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astro-ph/9703022
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Jon Loveday
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The local space density of dwarf galaxies
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astro-ph
|
We estimate the luminosity function of field galaxies over a range of ten
magnitudes (-22 < M_{B_J} < -12 for H_0 = 100 km/s/Mpc) by counting the number
of faint APM galaxies around Stromlo-APM redshift survey galaxies at known
distance. The faint end of the luminosity function rises steeply at M_{B_J}
\approx -15, implying that the space density of dwarf galaxies is at least two
times larger than predicted by a Schechter function with flat faint-end slope.
Such a high abundance of dwarf galaxies at low redshift can help explain the
observed number counts and redshift distributions of faint galaxies without
invoking exotic models for galaxy evolution.
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astro-ph/9408054
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Dong Lai
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Hydrodynamics of Coalescing Binary Neutron Stars: Ellipsoidal Treatment
|
astro-ph
|
We employ an approximate treatment of dissipative hydrodynamics in three
dimensions to study the coalescence of binary neutron stars driven by the
emission of gravitational waves. The stars are modeled as compressible
ellipsoids obeying a polytropic equation of state; all internal fluid
velocities are assumed to be linear functions of the coordinates. The
hydrodynamic equations then reduce to a set of coupled ordinary differential
equations for the evolution of the principal axes of the ellipsoids, the
internal velocity parameters and the binary orbital parameters. Gravitational
radiation reaction and viscous dissipation are both incorporated. We set up
exact initial binary equilibrium configurations and follow the transition from
the quasi-static, secular decay of the orbit at large separation to the rapid
dynamical evolution of the configurations just prior to contact. A
hydrodynamical instability resulting from tidal interactions significantly
accelerates the coalescence at small separation, leading to appreciable radial
infall velocity and tidal lag angles near contact. This behavior is reflected
in the gravitational waveforms and may be observable by gravitational wave
detectors under construction.
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astro-ph/0305338
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Sara Seager
|
The Significance of the Sodium Detection in the Extrasolar Planet
HD209458b Atmosphere
|
astro-ph
|
The Hubble Space Telescope (HST) detection of an extrasolar planet atmosphere
in 2001 was a landmark step forward for the characterization of extrasolar
planets. HST detected the trace element sodium, via the neutral atomic
resonance doublet at 593 nm, in the transiting extrasolar giant planet
HD209458b. In this paper I discuss the significance of this first ever
extrasolar planet atmosphere detection. I explain how the sodium measurement
can be used as a constraint on HD209458b atmosphere models and review recent
interpretations of the lower-than-expected sodium line strength.
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astro-ph/0404065
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Orsola De Marco
|
First Evidence of Circumstellar Disks around Blue Straggler Stars
|
astro-ph
|
We present an analysis of optical HST/STIS and HST/FOS spectroscopy of 6 blue
stragglers found in the globular clusters M3, NGC6752 and NGC6397. These stars
are a subsample of a set of ~50 blue stragglers and stars above the main
sequence turn-off in four globular clusters which will be presented in an
forthcoming paper. All but the 6 stars presented here can be well fitted with
non-LTE model atmospheres. The 6 misfits, on the other hand, possess Balmer
jumps which are too large for the effective temperatures implied by their
Paschen continua. We find that our data for these stars are consistent with
models only if we account for extra absorption of stellar Balmer photons by an
ionized circumstellar disk. Column densities of HI and CaII are derived as are
the the disks' thicknesses. This is the first time that a circumstellar disk is
detected around blue stragglers. The presence of magnetically-locked disks
attached to the stars has been suggested as a mechanism to lose the large
angular momentum imparted by the collision event at the birth of these stars.
The disks implied by our study might not be massive enough to constitute such
an angular momentum sink, but they could be the leftovers of once larger disks.
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0811.3654
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Alessandro Baldi
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Chandra observations of the galaxy group AWM 5: cool core re-heating and
thermal conduction suppression
|
astro-ph
|
We present an analysis of a 40 ksec Chandra observation of the galaxy group
AWM 5. It has a small ($\sim8$ kpc) dense cool core with a temperature of
$\sim1.2$ keV and the temperature profile decreases at larger radii, from
$\sim3.5$ keV just outside the core to $\sim2$ keV at $\sim300$ kpc from the
center. The abundance distribution shows a "hole" in the central $\sim10$ kpc,
where the temperature declines sharply. An abundance of at least a few times
solar is observed $\sim15-20$ kpc from the center. The deprojected electron
density profile shows a break in slope at $\sim13$ kpc and can be fit by two
$\beta$-models, with $\beta=0.72_{-0.11}^{+0.16}$ and $r_c=5.7_{-1.5}^{+1.8}$
kpc, for the inner part, and $\beta=0.34\pm0.01$ and $r_c=31.3_{-5.5}^{+5.8}$
kpc, for the outer part. The mass fraction of hot gas is fairly flat in the
center and increases for $r>30$ kpc up to a maximum of $\sim6.5%$ at $r\sim380$
kpc. The gas cooling time within the central 30 kpc is smaller than a Hubble
time, although the temperature only declines in the central $\sim8$ kpc region.
This discrepancy suggests that an existing cooling core has been partially
re-heated. In particular, thermal conduction could have been a significant
source of re-heating. In order for heating due to conduction to balance cooling
due to emission of X-rays, the conductivity must be suppressed by a large
factor (at least $\sim100$). Past AGN activity (still visible as a radio source
in the center of the group) is however the most likely source that re-heated
the central regions of AWM 5.
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astro-ph/0512475
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Amata Mercurio
|
Shapley Optical Survey. I: Luminosity Functions in the Supercluster
Environment
|
astro-ph
|
We present the Shapley Optical Survey, a photometric study covering a 2 deg^2
region of the Shapley Supercluster core at z ~ 0.05 in two bands (B and R). The
galaxy sample is complete to B=22.5 (>M^*+6, N_{gal}=16588), and R=22.0
(>M^*+7, N_{gal}=28008). The galaxy luminosity function cannot be described by
a single Schechter function due to dips apparent at B ~ 17.5 (M_B ~ -19.3) and
R ~ 17.0 (M_R ~ -19.8) and the clear upturn in the counts for galaxies fainter
than B and R ~18 mag. We find, instead, that the sum of a Gaussian and a
Schechter function, for bright and faint galaxies respectively, is a suitable
representation of the data. We study the effects of the environment on the
photometric properties of galaxies, deriving the galaxy luminosity functions in
three regions selected according to the local galaxy density, and find a marked
luminosity segregation, in the sense that the LF faint-end is different at more
than 3sigma confidence level in regions with different densities. In addition,
the luminosity functions of red and blue galaxy populations show very different
behaviours: while red sequence counts are very similar to those obtained for
the global galaxy population, the blue galaxy luminosity functions are well
described by a single Schechter function and do not vary with the density. Such
large environmentally-dependent deviations from a single Schechter function are
difficult to produce solely within galaxy merging or suffocation scenarios.
Instead the data support the idea that mechanisms related to the cluster
environment, such as galaxy harassment or ram-pressure stripping, shape the
galaxy LFs by terminating star-formation and producing mass loss in galaxies at
\~M^*+2, a magnitude range where blue late-type spirals used to dominate
cluster populations, but are now absent.
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0711.4856
|
David Spiegel
|
Habitable Climates
|
astro-ph
|
According to the standard liquid-water definition, the Earth is only
partially habitable. We reconsider planetary habitability in the framework of
energy-balance models, the simplest seasonal models in physical climatology, to
assess the spatial and temporal habitability of Earth-like planets. We quantify
the degree of climatic habitability of our models with several metrics of
fractional habitability. Previous evaluations of habitable zones may have
omitted important climatic conditions by focusing on close Solar System
analogies. For example, we find that model pseudo-Earths with different
rotation rates or different land-ocean fractions have fractional habitabilities
that differ significantly from that of the Earth itself. Furthermore, the
stability of a planet's climate against albedo-feedback snowball events
strongly impacts its habitability. Therefore, issues of climate dynamics may be
central in assessing the habitability of discovered terrestrial exoplanets,
especially if astronomical forcing conditions are different from the moderate
Solar System cases.
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0803.3631
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Katie Chynoweth
|
Neutral Hydrogen Clouds in the M81/M82 Group
|
astro-ph
|
We have observed a 3 degree x 3 degree area centered on the M81/M82 group of
galaxies using the Robert C. Byrd Green Bank Telescope (GBT) in a search for
analogs to the High Velocity Clouds (HVCs) of neutral hydrogen found around our
galaxy. The velocity range from -605 to -85 km/s and 25 to 1970 km/s was
searched for HI clouds. Over the inner 2 degrees x 2 degrees the 7-sigma
detection threshold was 9.6 x 10^5 M_sun. We detect 5 previously unknown HI
clouds associated with the group, as well as numerous associated filamentary HI
structures, all lying in the range -105 < V_helio < +280 km/s. From the small
angular distance of the clouds to group members, and the small velocity
difference between group members and clouds, we conclude that the clouds are
most likely relics of ongoing interactions between galaxies in the group.
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astro-ph/0511675
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Stefan Gottloeber
|
Halo Shapes and their Relation to Environment
|
astro-ph
|
Using high resolution DM simulations we study the shape of dark matter halos.
Halos become more spherical with decreasing mass. This trend is even more
pronounced for the inner part of the halo. Angular momentum and shape are
correlated. The angular momenta of neighboring halos are correlated.
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astro-ph/0402040
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Marina Orio
|
A New View of the Supersoft X-Ray Source Cal 87 Observed with XMM-Newton
|
astro-ph
|
Cal 87 was observed with XMM-Newton in April of 2003. The source shows a rich
emission line spectrum, where lines can be identified if they are red-shifted
by 700-1200 km/s. These lines seem to have been emitted in a wind from the
system. The eclipse is observed to be shifted in phase by 0.03 phi(orb), where
phi(orb) is the phase of the optical light curve.
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0806.0113
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Jean-Marc Casandjian
|
A revised catalogue of EGRET gamma-ray sources
|
astro-ph
|
We present a catalog of point gamma-ray sources detected by the EGRET
detector aboard the Compton Gamma Ray Observatory. We have used the whole
gamma-ray dataset of reprocessed photons at energies above 100 MeV together
with new Galactic interstellar emission models based on recent CO, HI, dark
gas, and interstellar radiation field data. Two different assumptions have been
used for the cosmic-ray distribution in the Galaxy to explore the resulting
systematic uncertainties in source detection and characterization. We have used
the same 2-dimensional maximum-likelihood detection method as for the 3rd EGRET
catalogue. The revised catalogue lists 188 sources, 14 of which are marked as
confused, compared to the 271 entries of the 3rd EGRET (3EG) catalogue. 107
former sources have not been confirmed because of the additional structure in
the interstellar background. The vast majority of them were unidentified and
marked as possibly extended or confused in the 3EG catalogue. In particular, we
do not confirm most of the 3EG sources associated with the local clouds of the
Gould Belt. Alternatively, we find 30 new sources with no 3EG counterpart. The
new error circles for the confirmed 3EG sources largely overlap the previous
ones, but several counterparts of particular interest that had been discussed
in the litterature, such as Sgr A*, radiogalaxies and several microquasars are
now found outside the error circles. We have cross-correlated the source
positions with a large number of radio pulsars, pulsar wind nebulae, supernova
remnants, OB associations, blazars and flat radiosources and we find a
surprising large number of sources (87) at all latitudes with no counterpart
among the potential gamma-ray emitters.
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astro-ph/9611007
| null |
Near-Infrared Synthetic Spectra of Elliptical Galaxies
|
astro-ph
|
We present the first results from our population synthesis models of
elliptical galaxies. In this paper, we concentrate upon the near-infrared
region of the spectrum (0.7-3 um). We model elliptical galaxies as coeval,
single-metallicity stellar populations. Integrated spectra are constructed from
synthetic spectra of the stars which lie along 6, 10 and 16 Gyr isochrones
having [Fe/H]=+0.39, 0.00 and -0.47. The V-K and H-K colors and CO indices
measured from our synthetic, integrated spectra are found to agree well with
E/S0 galaxy photometry, but the J-K colors of the models are redder than the
galaxies at a given V-K. We attribute this discrepancy to the oscillator
strengths we employ for spectral lines in the phi system of TiO. We have
probably overestimated the strengths of these lines, causing our J-band
magnitudes to be too faint and our model J-K colors too red.
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astro-ph/0609323
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Jonathan Slavin
|
Evidence for a High Carbon Abundance in the Local Interstellar Cloud
|
astro-ph
|
The nature of the Local Interstellar Cloud (LIC) is highly constrained by the
combination of in situ heliospheric and line-of-sight data towards nearby
stars. We present a new interpretation of the LIC components of the absorption
line data towards epsilon CMa, based on recent atomic data that include new
rates for the Mg+ to Mg0 dielectronic recombination rate, and using in situ
measurements of the temperature and density of neutral helium inside of the
heliosphere. With these data we are able to place interesting limits on the gas
phase abundance of carbon in the LIC. If the C/S abundance ratio is solar, ~20,
then no simultaneous solution exists for the N(Mg I), N(Mg II), N(C II) and N(C
II*) data. The combined column density and in situ data favor an abundance
ratio A(C)/A(S) = 47 +22 -26.
We find that the most probable gas phase C abundance is in the range 400 to
800 ppm with a lower limit of ~330. We speculate that such a supersolar
abundance could have come to be present in the LIC via destruction of decoupled
dust grains. Similar enhanced C/H ratios are seen in very low column density
material, N(H) < 10^19 cm^-2, towards several nearby stars.
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astro-ph/0107265
|
Gurzadyan
|
Recovering the Internal Dynamics and the Shapes of Galaxy Clusters:
Virgo Cluster
|
astro-ph
|
We describe a method for recovering of the substructure, internal dynamics
and geometrical shapes of clusters of galaxies. Applying the method to the
Virgo cluster, we, first, reveal the substructure of the central 4 arc degree
field of the Virgo cluster by means of S-tree technique. The existence of three
main subgroups of galaxies is revealed and their dynamical characteristics are
estimated. Then, using the previously suggested technique (Gurzadyan and Rauzy
1997), the bulk flow velocities of the subgroups are evaluated based on the
distribution of the redshifts of the galaxies. The results enable us also to
obtain a secure indication of the elongation of the Virgo cluster and its
positional inclination.
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astro-ph/0610010
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Zhiping Jin
|
A two component jet model for the X-ray afterglow flat segment in short
GRB 051221A
|
astro-ph
|
In the double neutron star merger or neutron star-black hole merger model for
short GRBs, the outflow launched might be mildly magnetized and neutron rich.
The magnetized neutron-rich outflow will be accelerated by the magnetic and
thermal pressure and may form a two component jet finally, as suggested by
Vlahakis, Peng & K\"{o}nigl (2003). We show in this work that such a two
component jet model could well reproduce the multi-wavelength afterglow
lightcurves, in particular the X-ray flat segment, of short GRB 051221A. In
this model, the central engine need not to be active much longer than the
prompt $\gamma-$ray emission.
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0712.0389
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Andrea Kunder
|
The Extinction Toward the Galactic Bulge from RR Lyrae Stars
|
astro-ph
|
We present mean reddenings toward 3525 RR0 Lyrae stars from the Galactic
bulge fields of the MACHO Survey. These reddenings are determined using the
color at minimum $V$-band light of the RR0 Lyrae stars themselves and are found
to be in general agreement with extinction estimates at the same location
obtained from other methods. Using 3256 stars located in the Galactic Bulge, we
derive the selective extinction coefficient $R_{V,VR}=A_V/E(V-R) = 4.3 \pm
0.2$. This value is what is expected for a standard extinction law with
$R_{V,BV} = 3.1 \pm 0.3$.
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astro-ph/0111194
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Nick Gnedin
|
Matter Power Spectrum from the Lyman-Alpha Forest: Myth or Reality?
|
astro-ph
|
We investigate possible systematic errors in the recent measurement of the
matter power spectrum from the Lyman-alpha forest by Croft et al. (2001). We
find that for a large set of prior cosmological models the Croft et al. result
holds quite well, with systematic errors being comparable to random ones, when
a dependence of the recovered matter power spectrum on the cosmological
parameters at z~3 is taken into account. We find that peculiar velocities cause
the flux power spectrum to be smoothed over about 100-300 km/s, dependng on
scale. Consequently, the recovered matter power spectrum is a smoothed version
of the underlying true power spectrum. Uncertainties in the recovered power
spectrum are thus correlated over about 100-300 km/s. As a side effect, we find
that residual fluctuations in the ionizing background, while having almost no
effect on the recovered matter power spectrum, significantly bias estimates of
the baryon density from the Lyman-alpha forest data. We therefore conclude that
the Croft et al. result provides a powerful new constraint on cosmological
parameters and models of structure formation.
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astro-ph/0610457
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Christian Zier
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Merging of a massive black hole binary II
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astro-ph
|
In this paper, the second in a series of two, we justify two important
assumptions on which the result is based that in course of a galaxy merger the
slingshot ejection of bound stars is sufficiently efficient to allow a
supermassive black hole binary to merge. A steep cusp with a power law index of
2.5 to 3 is required which is as massive as the binary and surrounds the BHs
when the binary becomes hard. This cusp is probably formed when both clusters,
surrounding each black hole, merge and combine with the matter funneled into
the center. We find this profile to be in agreement with observed post-merger
distributions after the cusp has been destroyed. The time dependency we derive
for the merger predicts that stalled black holes, if they exist at all, will
preferably be found in less than about 0.2 pc distance. To test this prediction
we compute the current semimajor axis of 12 candidates of ongoing mergers. We
find all binaries unambiguously to be already in the last phase when they decay
due to the emission of gravitational waves. Therefore, in striking
contradiction with predictions of a depleted loss-cone, the abscence of even a
single source in the slingshot phase strongly supports our previous and current
results: Binaries merge due to slingshot ejection of stars which have been
funneled into the central regions in course of a galaxy collision.
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astro-ph/0403138
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Masaki Mori
|
Gamma-ray Spectra due to Cosmic-ray Interactions with Dense Gas Clouds
|
astro-ph
|
Gamma-ray spectra from cosmic-ray proton and electron interactions with dense
gas clouds have been calculated using a Monte Carlo event simulation code,
GEANT4. Such clouds are postulated as a possible form of baryonic dark matter
in the Universe. The simulation fully tracks the cascade and transport
processes that are important in a dense medium, and the resulting gamma-ray
spectra are computed as a function of cloud column-density. These calculations
are used for predicting the Galactic diffuse gamma-ray spectrum that may be
contributed by baryonic dark matter; the results are compared with data from
the EGRET instrument, and used to constrain the fraction of Galactic dark
matter that may be in the form of dense gas clouds. In agreement with previous
authors, we find useful constraints on the fraction of Galactic dark matter
that may be in the form of low column-density clouds ($\Sigma \la 10 g
cm^{-2}$). However, this fraction rises steeply in the region $\Sigma \sim 10^2
g cm^{-2}$, and for $\Sigma \ga 200 g cm^{-2}$ we find that baryonic dark
matter models are virtually unconstrained by the existing gamma-ray data.
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astro-ph/0105037
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Mario van den Ancker
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The Young Stellar Group Associated with HD 199143
|
astro-ph
|
Recently, several groups of young stars in the solar neighborhood have been
discovered. Given their proximity, these systems are ideally suited for
detailed studies of star and planet formation. Here we report on a group of
young stars associated with the bright F8V star HD 199143. At a distance of
only 48 pc, this is the closest YSO group containing a classical T Tauri star
(HD 358623; K7-M0e). New ground-based mid-infrared data shows that both HD
199143 and HD 358623 have large infrared excesses due to circumstellar disks. A
systematic search for new members of this Capricornius association has yielded
four new probable members, which we use to derive an age of 5-10 Myr for the
group as a whole.
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astro-ph/9903196
|
Carlos Gutierrez
|
The Tenerife Cosmic Microwave Background Maps: Observations and First
Analysis
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astro-ph
|
The results of the Tenerife Cosmic Microwave Background (CMB) experiments are
presented. These observations cover 5000 and 6500 square degrees on the sky at
10 and 15 GHz respectively centred around Dec.~ +35 degrees. The experiments
are sensitive to multipoles l=10-30 which corresponds to the Sachs-Wolfe
plateau of the CMB power spectra. The sensitivity of the results are ~31 and
\~12 microK at 10 and 15 GHz respectively in a beam-size region (5 degrees
FWHM). The data at 15 GHz show clear detection of structure at high Galactic
latitude; the results at 10 GHz are compatible with these, but at lower
significance. A likelihood analysis of the 10 and 15 GHz data at high Galactic
latitude, assuming a flat CMB band power spectra gives a signal Delta
T_l=30+10-8 microK (68 % C.L.). Including the possible contaminating effect due
to the diffuse Galactic component, the CMB signal is Delta T_l=30+15-11 microK.
These values are highly stable against the Galactic cut chosen. Assuming a
Harrison-Zeldovich spectrum for the primordial fluctuations, the above values
imply an expected quadrupole Q_RMS-PS=20+10-7 microK which confirms previous
results from these experiments, and which are compatible with the COBE DMR.
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astro-ph/0503204
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Harald Ebeling
|
The Origin of the Spatial Distribution of X-ray luminous AGN in Massive
Galaxy Clusters
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astro-ph
|
We study the spatial distribution of a 95% complete sample of 508 X-ray point
sources (XPS) detected in the 0.5-2.0 keV band in Chandra ACIS-I observations
of 51 massive galaxy clusters found in the MACS survey. Covering the redshift
range z=0.3-0.7, our cluster sample is statistically complete and comprises all
MACS clusters with X-ray luminosities in excess of 4.5 x 10^44 erg/s (0.1-2.4
keV, h_0=0.7, LCDM). Also studied are 20 control fields that do not contain
clusters. We find the XPS surface density, computed in the cluster restframe,
to exhibit a pronounced excess within 3.5 Mpc of the cluster centers. The
excess, believed to be caused by AGN in the cluster, is significant at the 8.0
sigma confidence level compared to the XPS density observed at the field edges.
No significant central excess is found in the control fields. To investigate
the physical origin of the AGN excess, we study the radial AGN density profile
for a subset of 24 virialized clusters. We find a pronounced central spike
(r<0.5 Mpc), followed by a depletion region at about 1.5 Mpc, and a broad
secondary excess centered at approximately the virial radius of the host
clusters (~2.5 Mpc). We present evidence that the central AGN excess reflects
increased nuclear activity triggered by close encounters between infalling
galaxies and the giant cD-type elliptical occupying the very cluster center. By
contrast, the secondary excess at the cluster-field interface is likely due to
black holes being fueled by galaxy mergers. In-depth spectroscopic and
photometric follow-up observations of the optical counterparts of the XPS in a
subset of our sample are being conducted to confirm this picture.
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0711.3022
|
Lisa Gerhardt
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Search for Ultra High-Energy Neutrinos with AMANDA-II
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astro-ph
|
A search for diffuse neutrinos with energies in excess of 10^5 GeV is
conducted with AMANDA-II data recorded between 2000 and 2002. Above 10^7 GeV,
the Earth is essentially opaque to neutrinos. This fact, combined with the
limited overburden of the AMANDA-II detector (roughly 1.5 km), concentrates
these ultra high-energy neutrinos at the horizon. The primary background for
this analysis is bundles of downgoing, high-energy muons from the interaction
of cosmic rays in the atmosphere. No statistically significant excess above the
expected background is seen in the data, and an upper limit is set on the
diffuse all-flavor neutrino flux of E^{-2} $\Phi$_{90%CL} < 2.7 $\times$
10^{-7} GeV cm^{-2} s^{-1} sr^{-1} valid over the energy range of 2 $\times$
10^5 GeV to 10^9 GeV. A number of models which predict neutrino fluxes from
active galactic nuclei are excluded at the 90% confidence level.
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astro-ph/0612273
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Ian McHardy
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Active Galactic Nuclei as scaled-up Galactic black holes
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astro-ph
|
A long-standing question is whether active galactic nuclei (AGN) vary like
Galactic black hole systems when appropriately scaled up by mass (refs 1-3). If
so, we can then determine how AGN should behave on cosmological timescales by
studying the brighter and much faster varying Galactic systems. As X-ray
emission is produced very close to the black holes, it provides one of the best
diagnostics of their behaviour. A characteristic timescale, which potentially
could tell us about the mass of the black hole, is found in the X-ray
variations from both AGN and Galactic black holes (refs 1-6), but whether it is
physically meaningful to compare the two has been questioned (ref 7). Here we
report that, after correcting for variations in the accretion rate, the
timescales can be physically linked, revealing that the accretion process is
exactly the same for small and large black holes. Strong support for this
linkage comes, perhaps surprisingly, from the permitted optical emission lines
in AGN whose widths (in both broad-line AGN and narrow-emission-line Seyfert 1
galaxies) correlate strongly with the characteristic X-ray timescale, exactly
as expected from the AGN black hole masses and accretion rates. So AGN really
are just scaled-up Galactic black holes.
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astro-ph/0405616
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Joseph L. Hora
|
The Infrared Array Camera (IRAC) for the Spitzer Space Telescope
|
astro-ph
|
The Infrared Array Camera (IRAC) is one of three focal plane instruments in
the Spitzer Space Telescope. IRAC is a four-channel camera that obtains
simultaneous broad-band images at 3.6, 4.5, 5.8, and 8.0 microns. Two nearly
adjacent 5.2x5.2 arcmin fields of view in the focal plane are viewed by the
four channels in pairs (3.6 and 5.8 microns; 4.5 and 8 microns). All four
detector arrays in the camera are 256x256 pixels in size, with the two shorter
wavelength channels using InSb and the two longer wavelength channels using
Si:As IBC detectors. IRAC is a powerful survey instrument because of its high
sensitivity, large field of view, and four-color imaging. This paper summarizes
the in-flight scientific, technical, and operational performance of IRAC.
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astro-ph/9910425
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Eric M. Schlegel
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X-ray Detection of SN1994W in NGC 4041?
|
astro-ph
|
Optical spectra of SN1994W in NGC 4041 revealed the presence of a dense (N_e
> 10^8 cm^-3) circumstellar shell. An observation with the ROSAT HRI detected a
source, with a 0.2-2.4 keV luminosity of ~8x10^39 erg/sec, coincident with the
position of SN1994W to within 1''.4. The positional coincidence plus the
optical evidence for a dense circumstellar shell support the identification of
the X-ray source as SN1994W.
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astro-ph/0701734
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Prashanth Jaikumar
|
Quark Stars: Features and Findings
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astro-ph
|
Under extreme conditions of temperature and/or density, quarks and gluons are
expected to undergo a deconfinement phase transition. While this is an
ephemeral phenomenon at the ultra-relativistic heavy-ion collider (BNL-RHIC),
quark matter may exist naturally in the dense interior of neutron stars.
Herein, we present an appraisal of the possible phase structure of dense quark
matter inside neutron stars, and the likelihood of its existence given the
current status of neutron star observations. We conclude that quark matter
inside neutron stars cannot be dismissed as a possibility, although recent
observational evidence rules out most soft equations of state.
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No Label
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astro-ph/9410048
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Dave Bennett
|
Is There Evidence for Repeating Gamma Ray Bursters in the BATSE Data?
|
astro-ph
|
The possibility that classical gamma ray bursts (GRB) occasionally repeat
from the same locations on the sky provides a critical test of GRB models.
There is currently some controversy about whether there is evidence for burst
repetition in the BATSE data. We introduce a gamma ray burst ``pair matching"
statistic that can be used to search for a repeater signal in the BATSE data.
The pair matching statistic is built upon the reported position errors for each
burst and is more sensitive than previously used statistics at detecting faint
repeating bursts or multiple burst recurrences. It is also less likely to
produce (false) evidence of burst repetition due to correlations in the
positions that are inconsistent with repeating bursters. We find that the
excesses in ``matched" and ``antipodal" pairs seen with other statistics are
caused by an excess of pairs with a separation smaller than their error bars
would indicate. When we consider all separations consistent with the error
bars, no significant signal remains. We conclude that the publicly available
BATSE 1B and 2B data sets contain no evidence for repeating gamma ray bursters.
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astro-ph/0307454
|
Fulgencio Garc\'ia Dr.
|
Collimated Outflow Formation via Binary Stars. 3-D Simulations of AGB
Wind and Disk Wind Interactions
|
astro-ph
|
We present three-dimensional hydrodynamic simulations of the interaction of a
slow wind from an asymptotic giant branch(AGB) star and a jet blown by an
orbiting companion. The jet or "Collimated Fast Wind" is assumed to originate
from an accretion disk which forms via Bondi accretion of the AGB wind or Roche
lobe overflow. We present two distinct regimes in the wind-jet interaction
determined by the ratio of the AGB wind to jet momentum flux. Our results show
that when the wind momentum flux overwhelms the flux in the jet a more
dis-ordered outflow outflow results with the jet assuming a corkscrew pattern
and multiple shock structures driven into the AGB wind. In the opposite regime
the jet dominates and will drive a highly collimated narrow waisted outflow. We
compare our results with scenarios described by Soker & Rappaport (2000) and
extrapolate the structures observed in PNe and Symbiotic stars.
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No Label
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0707.1782
|
Jerome Petri
|
Magnetic reconnection at the termination shock in a striped pulsar wind
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astro-ph
|
Most of the rotational luminosity of a pulsar is carried away by a
relativistic magnetised wind in which the matter energy flux is negligible
compared to the Poynting flux. Near the equatorial plane of an obliquely
rotating pulsar magnetosphere, the magnetic field reverses polarity with the
pulsar period, forming a wind with oppositely directed field lines. This
structure is called a striped wind; dissipation of alternating fields in the
striped wind is the object of our study.
The aim of this paper is to study the conditions required for magnetic energy
release at the termination shock of the striped pulsar wind. Magnetic
reconnection is considered via analytical methods and 1D relativistic PIC
simulations.
An analytical condition on the upstream parameters for partial and full
magnetic reconnection is derived from the conservation laws of energy, momentum
and particle number density across the relativistic shock. Furthermore, by
using a 1D relativistic PIC code, we study in detail the reconnection process
at the termination shock.
We found a very simple criterion for dissipation of alternating fields at the
termination shock, depending on the upstream parameters of the flow. 1D
relativistic PIC simulations are in agreement with our criterion.
Thus, alternating magnetic fields annihilate easily at relativistic highly
magnetised shocks.
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No Label
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astro-ph/9612117
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Joe Silk
|
Feedback, Disk Self-regulation and Galaxy Formation
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astro-ph
|
Self-regulation of star formation in disks is controlled by two dimensionless
parameters: the Toomre parameter for gravitational instability and the porosity
of the interstellar medium to supernova remnant-heated gas. An interplay
between these leads to expressions for the gas fraction and star formation
efficiency in disks, and to a possible explanation of the Tully-Fisher
relation. I further develop feedback arguments that arise from the impact of
massive star formation and death on protogalaxies in order to account for the
characteristic luminosity of a galaxy and for early winds from forming
spheroids.
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No Label
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astro-ph/0210365
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Makoto Matsumiya
|
Cosmic Inversion II --An iterative method for reproducing the primordial
spectrum from the CMB data--
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astro-ph
|
Pursuing the original idea proposed in our previous paper (Paper I), we
improve the method to determine the shape of the initial curvature perturbation
spectrum $P(k)$ from the CMB data. The thickness of the last scattering surface
(LSS) and the integrated Sachs-Wolfe (ISW) effect, which we neglect in Paper I,
are taken into account and an iterative method is newly developed. The new
method can reproduce the primordial power spectra with a high accuracy, given
the correct values of the cosmological parameters. Conversely, there appear
spurious peaks and dips in the reconstructed power spectrum if we use the
cosmological parameters slightly different from the true values, while there
appear regions of negative $P(k)$ in some cases if we use substantially
different values. In other words, the tacit assumption that the cosmological
parameters can be determined for an assumed initial spectrum is verified by our
reconstruction method. In addition, it could be a new tool to constrain the
cosmological parameters without recourse to models of the primordial power
spectra.
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No Label
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astro-ph/0108377
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Sergio A. Ilovaisky
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RX J1643.7+3402 : a new bright cataclysmic variable
|
astro-ph
|
We report the discovery of a new bright (V$\sim$12.6) cataclysmic variable
star identified with the ROSAT X-ray source RX J1643.7+3402. Spectroscopic and
photometric observations show it to be a novalike variable sharing some of the
characteristics of the SW Sex sub-class of novalike CVs. The spectroscopic
period may be either 2\fh575 or 2\fh885, within the period "gap." A photometric
modulation with a probable period of 2\fh595 and an amplitude of $\sim$ 0.1 mag
in V is present on most nights and could be either a "positive" or a "negative"
superhump modulation (depending on the exact spectroscopic period), indicating
the presence of a precessing accretion disk in this system. Rapid variations of
0.1 to 0.2 mag amplitude in V repeat with a time scale of $\sim$ 15 min.
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astro-ph/0411326
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Ryo Yamazaki
|
A Spatial and Spectral Study of Nonthermal Filaments in Historical
Supernova Remnants: Observational Results with Chandra
|
astro-ph
|
The outer shells of young supernova remnants (SNRs) are the most plausible
acceleration sites of high-energy electrons with the diffusive shock
acceleration (DSA) mechanism. We studied spatial and spectral properties close
to the shock fronts in four historical SNRs (Cas A, Kepler's remnant, Tycho's
remnant, and RCW 86) with excellent spatial resolution of {\it Chandra}. In all
of the SNRs, hard X-ray emissions were found on the rims of the SNRs, which
concentrate in very narrow regions (so-called "filaments"); apparent scale
widths on the upstream side are below or in the order of the point spread
function of {\it Chandra}, while 0.5--40 arcsec (0.01--0.4 pc) on the
downstream side with most reliable distances. The spectra of these filaments
can be fitted with both thermal and nonthermal (power-law and {\tt SRCUT})
models. The former requires unrealistic high temperature ($\ga$2 keV) and low
abundances ($\la$1 solar) for emission from young SNRs and may be thus
unlikely. The latter reproduces the spectra with best-fit photon indices of
2.1--3.8, or roll-off frequencies of (0.1--28)$\times 10^{17}$ Hz, which
reminds us of the synchrotron emission from electrons accelerated via DSA. We
consider various physical parameters as functions of the SNR age, including the
previous results on SN 1006 \citep{bamba2003b}; the filament width on the
downstream side increases with the SNR age, and the spectrum becomes softer
keeping a nonthermal feature. It was also found that a function, that is the
roll-off frequency divided by the square of the scale width on the downstream
side, shows negative correlation with the age, which might provide us some
information on the DSA theory.
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astro-ph/0105075
|
Chanial
|
The first ISO ERO: a dusty quasar at z = 1.5
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astro-ph
|
We report the discovery of an extremely red object (ERO) in a medium-deep
ISOCAM extragalactic survey. The object is also a radio source. Subsequent VLT
NIR spectroscopy revealed a prominent Halpha line giving a redshift of 1.5. We
present the spectrum and photometric data points and discuss evidence that ISO
J1324-2016 is a quasar harbouring a significant amount of very hot dust.
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astro-ph/9401016
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Avi Loeb
|
Origin of Quasar Progenitors from the Collapse of Low-Spin Cosmological
Perturbations
|
astro-ph
|
We show that seeds for quasar black holes could have originated from the
initial cosmological collapse of overdense regions with unusually small
rotation. The gas in these rare regions collapses into a compact disk that
shrinks on a short viscous time scale. Using an analytical model, we calculate
the low-spin tail of the probability distribution of angular momenta for
objects that collapse out of a Gaussian random field of initial density
perturbations. The population of low-spin systems is significant for any viable
power spectrum of primordial density perturbations. Most objects form just
above the cosmological Jeans mass (\sim 10^5 M_sun) at high redshifts z>10. In
the standard cold dark matter cosmology, the comoving density of 10^{6-7} M_sun
objects with viscous evolution times shorter than 10^{6-7} years is about
10^{-3} (h/0.5)^3 Mpc^{-3}, comparable to the local density of bright galaxies.
The seed black holes tend to reside within larger mass systems that collapse
later and supply the gas needed for the bright quasar activity.
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astro-ph/0301649
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Danuta Dobrzycka
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Ultraviolet spectroscopy of narrow coronal mass ejections
|
astro-ph
|
We present Ultraviolet Coronagraph Spectrometer (UVCS) observations of 5
narrow coronal mass ejections (CMEs) that were among 15 narrow CMEs originally
selected by Gilbert et al. (2001). Two events (1999 March 27, April 15) were
"structured", i.e. in white light data they exhibited well defined interior
features, and three (1999 May 9, May 21, June 3) were "unstructured", i.e.
appeared featureless. In UVCS data the events were seen as 4-13 deg wide
enhancements of the strongest coronal lines HI Ly-alpha and OVI (1032,1037 A).
We derived electron densities for several of the events from the Large Angle
Spectrometric Coronagraph (LASCO) C2 white light observations. They are
comparable to or smaller than densities inferred for other CMEs. We modeled the
observable properties of examples of the structured (1999 April 15) and
unstructured (1999 May 9) narrow CMEs at different heights in the corona
between 1.5 and 2 R(Sun). The derived electron temperatures, densities and
outflow speeds are similar for those two types of ejections. They were compared
with properties of polar coronal jets and other CMEs. We discuss different
scenarios of narrow CME formation either as a jet formed by reconnection onto
open field lines or CME ejected by expansion of closed field structures.
Overall, we conclude that the existing observations do not definitively place
the narrow CMEs into the jet or the CME picture, but the acceleration of the
1999 April 15 event resembles acceleration seen in many CMEs, rather than
constant speeds or deceleration observed in jets.
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astro-ph/9707112
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Youri Dabrowski
|
Effect of a Collapsing Cluster on the CMB temperature and Power Spectrum
|
astro-ph
|
We present a new model for the formation of spherically symmetric clusters in
an expanding Universe. Both the Universe and the collapsing cluster are
governed by the same pressure less fluid equations for which a uniform initial
density profile is assumed. A simple perturbation imposed on the initial
velocity field gives rise to an over-density which closely models real
clusters. The computation of photon paths allows us to evaluate the
gravitational effects imprinted on a Cosmic Microwave Background (CMB) photon
passing through such an evolving mass. We also consider the lensing properties
of collapsing clusters and investigate the effect of a population of such
clusters on the primordial microwave background power spectrum.
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No Label
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astro-ph/0612022
|
Carlo Schimd
|
Scalar-field quintessence by cosmic shear: CFHT data analysis and
forecasts for DUNE
|
astro-ph
|
A light scalar field, minimally or not-minimally coupled to the metric field,
is a well-defined candidate for the dark energy, overcoming the coincidence
problem intrinsic to the cosmological constant and avoiding the difficulties of
parameterizations. We present a general description of the weak gravitational
lensing valid for every metric theory of gravity, including vector and tensor
perturbations for a non-flat spatial metric. Based on this description, we
investigate two minimally-coupled scalar field quintessence models using
VIRMOS-Descart and CFHTLS cosmic shear data, and forecast the constraints for
the proposed space-borne wide-field imager DUNE.
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No Label
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astro-ph/9709084
|
Gian Luigi Granato
|
Modelling Intermediate Age and Old Stellar Populations in the Infrared
|
astro-ph
|
We have investigated the spectro-photometric properties of the Asymptotic
Giant Branch (AGB) stars and their contribution to the integrated infrared
emission in simple stellar populations (SSP). Adopting analytical relations
describing the evolution of these stars in the HR diagram and empirical
relations for the mass-loss rate and the wind terminal velocity, we were able
to model the effects of the dusty envelope around these stars, with a minimal
number of parameters. We computed isochrones at different age and initial metal
content. We compare our models with existing infrared colors of M giants and
Mira stars and with IRAS PSC data. Contrary to previous models, in the new
isochrones the mass-loss rate, which establishes the duration of the AGB phase,
also determines the spectral properties of the stars. The contribution of these
stars to the integrated light of the population is thus obtained in a
consistent way. We find that the emission in the mid infrared is about one
order of magnitude larger when dust is taken into account in an intermediate
age population, irrespective of the particular mixture adopted. The dependence
of the integrated colors on the metallicity and age is discussed, with
particular emphasis on the problem of age-metallicity degeneracy. We show that,
contrary to the case of optical or near infrared colors, the adoption of a
suitable pass-band in the mid infrared allows a fair separation of the two
effects. We suggest intermediate redshift elliptical galaxies as possible
targets of this method of solving the age-metallicity dilemma. The new SSP
models constitute a first step in a more extended study aimed at modelling the
spectral properties of the galaxies from the ultraviolet to the far infrared.
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astro-ph/9806064
|
Jean-Pierre Lasota
|
ADAFs -- Models, Observations and Problems
|
astro-ph
|
We review some of the properties of Advection-Dominated Accretion Flow (ADAF)
models and show that they successfully describe many astrophysical systems.
Despite these successful applications some fundamental problems still remain to
be solved, the most important one being the physics of the transition between
an ADAF and a geometrically thin Keplerian disc.
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No Label
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astro-ph/0609294
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Esteban Roulet
|
On the ultra-high energy cosmic ray horizon
|
astro-ph
|
We compute the ultra-high energy cosmic ray horizon, i.e. the distance up to
which cosmic ray sources may significantly contribute to the fluxes above a
certain threshold on the observed energies. We obtain results both for proton
and heavy nuclei sources.
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No Label
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astro-ph/9902306
|
Charles Dermer
|
The External Shock Model of Gamma-Ray Bursts: Three Predictions and a
Paradox Resolved
|
astro-ph
|
In the external shock model, gamma-ray burst (GRB) emissions are produced by
the energization and deceleration of a thin relativistic blast wave due to its
interactions with the circumburst medium (CBM). We study the physical
properties of an analytic function which describes temporally-evolving GRB
spectra in the limit of a smooth CBM with density n(x)\propto x^(-\eta), where
x is the radial coordinate. The hard-to-soft spectral evolution and the
intensity-hardness correlation of GRB peaks are reproduced. We predict that (1)
GRB peaks are aligned at high photon energies and lag at low energies according
to a simple rule; that (2) temporal indices at the leading edge of a GRB peak
display a well-defined shift with photon energy; and that (3) the change in the
spectral index values between the leading and trailing edges of a GRB peak
decreases at higher photon energies. The reason that GRBs are usually detected
with vF_v peaks in the 50 keV - several MeV range for detectors which trigger
on peak flux over a fixed time interval is shown to be a consequence of the
inverse correlation of peak flux and duration of the radiation emitted by
decelerating blast waves.
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astro-ph/0208478
|
Steven B. Kraemer
|
The Kinematics and Physical Conditions pf the Ionized Gas in Markarian
509. II. STIS Echelle Observations
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astro-ph
|
We present observations of the UV absorption lines in the luminous Seyfert 1
galaxy Mrk 509, obtained with the medium resolution (lambda/Delta-lambda ~
40,000) echelle gratings of the Space Telescope Imaging Spectrograph on the
Hubble Space Telescope. The spectra reveal the presence of eight kinematic
components of absorption in Ly-alpha, C IV, and N V, at radial velocities of
-422, -328, -259, -62, -22, +34, +124, and +210 km s^-1 with respect to an
emission-line redshift of z = 0.03440, seven of which were detected in an
earlier Far Ultraviolet Spectrographic Explorer (FUSE) spectrum. The component
at -22 km s^-1 also shows absorption by Si IV. The covering factor and velocity
width of the Si IV lines were lower than those of the higher ionization lines
for this component, which is evidence for two separate absorbers at this
velocity. We have calculated photoionization models to match the UV column
densities in each of these components. Using the predicted O VI column
densities, we were able to match the O VI profiles observed in the FUSE
spectrum. Based on our results, none of the UV absorbers can produce the X-ray
absorption seen in simultaneous Chandra observations; therefore, there must be
more highly ionized gas in the radial velocity ranges covered by the UV
absorbers.
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No Label
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astro-ph/0502131
|
Stuartt Corder
|
AB Aurigae Resolved: Evidence for Spiral Structure
|
astro-ph
|
We obtained high angular resolution (~2") images of the 13CO(J=1-0) line and
2.7 millimeter continuum emission, and slightly lower resolution images of
12CO(J=1-0) and C18O(J=1-0) line emission toward the Herbig Ae star AB Aurigae.
We resolve a circumstellar disk of diameter 780 AU (FWHM) with a velocity
pattern consistent with a purely rotational disk at inclination 21.5 degrees
and position angle 58.6 degrees. Using Keplerian disk models, we find a central
source dynamical mass of 2.8+-0.1 Msun and a cutoff radius of 615 AU for the
13CO emission. Inclination, mass, and radius determined from 12CO and C18O
observations agree with those values, given optical depth and abundance
effects. As a result of the high angular resolution of our observations, we
confirm the existence of spiral structure suggested by near-IR scattered light
images and show that the spiral arms represent density contrasts in the disk.
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astro-ph/9707226
|
H. M. Antia
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Estimate of solar radius from f-mode frequencies
|
astro-ph
|
Frequency and rotational splittings of the solar f-modes are estimated from
the GONG data. Contrary to earlier observations the frequencies of f-modes are
found to be close to the theoretically computed values for a standard solar
model. The f-mode being essentially a surface mode is a valuable diagnostic
probe of the properties of the solar surface, and also provides an independent
measure of solar radius. The estimated solar radius is found to be about 0.03%
less than what is traditionally used in construction of standard solar models.
If this decrease in solar radius is confirmed then the current solar models as
well as inversion results will need to be revised. The rotational splittings of
the f-modes yield an independent measure of the rotation rate near the solar
surface, which is compared with other measurements.
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No Label
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0803.0773
|
Paul Shankland
|
Further Constraints on the Presence of a Debris Disk in the Multiplanet
System Gliese 876
|
astro-ph
|
Using both the Very Large Array (VLA) at 7mm wavelength, and the Australia
Telescope Compact Array (ATCA) at 3mm, we have searched for microwave emission
from from cool dust in the extrasolar planetary system Gliese 876 (Gl 876).
Having detected no emission above our 3-sigma detection threshold of 135
microJy, we rule out any dust disk with either a mass greater than 0.0006 Earth
masses or less than ~250 AU across. This result improves on previous detection
aperture thresholds an order of magnitude greater, and it has some implications
for the dynamical modeling of the system. It also is consistent with the
Greaves et al. hypothesis that relates the presence of a debris disk to
close-in planets. Due to the dust-planetesimal relationship, our null result
may also provide a constraint on the population or composition of the dust and
small bodies around this nearby M dwarf.
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astro-ph/9812218
|
Narciso Benitez
|
Deep Imaging of AXJ2019+112: The Luminosity of a ``Dark Cluster''
|
astro-ph
|
We detect a distant cluster of galaxies centered on the QSO lens and luminous
X-ray source AXJ2019+112, a.k.a. ``The Dark Cluster'' (Hattori et al 1997).
Using deep V,I Keck images and wide-field K_s imaging from the NTT, a tight red
sequence of galaxies is identified within a radius of 0.2 h^{-1} Mpc of the
known z=1.01 elliptical lensing galaxy. The sequence, which includes the
central elliptical galaxy, has a slope in good agreement with the model
predictions of Kodama et al (1998) for z~1. We estimate the integrated
rest-frame luminosity of the cluster to be L_V > 3.2 x 10^{11}h^{-2}L_{\sun}
(after accounting for significant extinction at the low latitude of this
field), more than an order of magnitude higher than previous estimates. The
central region of the cluster is deconvolved using the technique of Magain,
Courbin & Sohy (1998), revealing a thick central arc coincident with an
extended radio source. All the observed lensing features are readily explained
by differential magnification of a radio loud AGN by a shallow elliptical
potential. The QSO must lie just outside the diamond caustic, producing two
images, and the arc is a highly magnified image formed from a region close to
the center of the host galaxy, projecting inside the caustic. The
mass--to--light ratio within an aperture of 0.4 h ^{-1} Mpc is M_x/L_V=
224^{+112}_{-78}h(M/L_V)_{\sun}, using the X-ray temperature. The strong lens
model yields a compatible value, M/L_V= 372^{+94}_{-94}h(M/L_V)_{\sun}, whereas
an independent weak lensing analysis sets an upper limit of M/L_V <520
h(M/L_V)_{\sun}, typical of massive clusters.
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No Label
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astro-ph/0611611
|
Sang Chul Kim
|
Near-Infrared Photometric Study of the Galactic Open Clusters NGC 1641
and NGC 2394 Based on 2MASS Data
|
astro-ph
|
We present JHKs near-infrared CCD photometric study for the Galactic open
clusters NGC 1641 and NGC 2394. These clusters have never been studied before,
and we provide, for the first time, the cluster parameters; reddening,
distance, metallicity and age. NGC 1641 is an old open cluster with age 1.6 +/-
0.2 Gyr, metallicity [Fe/H] = 0.0 +/- 0.2 dex, distance modulus (m-M)_0 = 10.4
+/- 0.3 mag (d = 1.2 +/- 0.2 kpc), and reddening E(B-V) = 0.10 +/- 0.05 mag.
The parameters for the other old open cluster NGC 2394 are estimated to be age
= 1.1 +/- 0.2 Gyr, [Fe/H] = 0.0 +/- 0.2 dex, (m-M)_0 = 9.1 +/- 0.4 mag (d = 660
+/- 120 pc), and E(B-V) = 0.05 +/- 0.10 mag. The metallicities and distance
values for these two old open clusters are consistent with the relation between
the metallicities and the Galactocentric distances of other old open clusters.
We find the metallicity gradient of 53 old open clusters including NGC 1641 and
NGC 2394 to be Delta [Fe/H]/Delta R_gc = -0.067 +/- 0.009 dex/kpc.
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0711.1105
|
Mark Vogelsberger
|
The fine-grained phase-space structure of Cold Dark Matter halos
|
astro-ph
|
We present a new and completely general technique for calculating the
fine-grained phase-space structure of dark matter throughout the Galactic halo.
Our goal is to understand this structure on the scales relevant for direct and
indirect detection experiments. Our method is based on evaluating the geodesic
deviation equation along the trajectories of individual DM particles. It
requires no assumptions about the symmetry or stationarity of the halo
formation process. In this paper we study general static potentials which
exhibit more complex behaviour than the separable potentials studied
previously. For ellipsoidal logarithmic potentials with a core, phase mixing is
sensitive to the resonance structure, as indicated by the number of independent
orbital frequencies. Regions of chaotic mixing can be identified by the very
rapid decrease in the real space density of the associated dark matter streams.
We also study the evolution of stream density in ellipsoidal NFW halos with
radially varying isopotential shape, showing that if such a model is applied to
the Galactic halo, at least $10^5$ streams are expected near the Sun. The most
novel aspect of our approach is that general non-static systems can be studied
through implementation in a cosmological N-body code. Such an implementation
allows a robust and accurate evaluation of the enhancements in annihilation
radiation due to fine-scale structure such as caustics. We embed the scheme in
the current state-of-the-art code GADGET-3 and present tests which demonstrate
that N-body discreteness effects can be kept under control in realistic
configurations.
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astro-ph/0212146
|
Massimo Ricotti
|
Dependence of the Inner DM Profile on the Halo Mass
|
astro-ph
|
I compare the density profile of dark matter (DM) halos in cold dark matter
(CDM) N-body simulations with 1 Mpc, 32 Mpc, 256 Mpc and 1024 Mpc box sizes. In
dimensionless units the simulations differ only for the initial power spectrum
of density perturbations. I compare the profiles when the most massive halos
are composed of about 10^5 DM particles. The DM density profiles of the halos
in the 1 Mpc box show systematically shallower cores with respect to the
corresponding halos in the 32 Mpc simulation that have masses, M_{dm}, typical
of the Milky Way and are fitted by a NFW profile. The DM density profiles of
the halos in the 256 Mpc box are consistent with having steeper cores than the
corresponding halos in the 32 Mpc simulation, but higher mass resolution
simulations are needed to strengthen this result. Combined, these results
indicate that the density profile of DM halos is not universal, presenting
shallower cores in dwarf galaxies and steeper cores in clusters. Physically the
result sustains the hypothesis that the mass function of the accreting
satellites determines the inner slope of the DM profile. In comoving
coordinates, r, the profile \rho_{dm} \propto 1/(X^\alpha(1+X)^{3-\alpha}),
with X=c_\Delta r/r_\Delta, r_\Delta is the virial radius and \alpha
=\alpha(M_{dm}), provides a good fit to all the DM halos from dwarf galaxies to
clusters at any redshift with the same concentration parameter c_\Delta ~ 7.
The slope, \gamma, of the outer parts of the halo appears to depend on the
acceleration of the universe: when the scale parameter is a=(1+z)^{-1} < 1, the
slope is \gamma ~ 3 as in the NFW profile, but \gamma ~ 4 at a > 1 when
\Omega_\Lambda ~ 1 and the universe is inflating.[abridged]
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0802.4154
|
Bruno Leibundgut
|
Supernovae and Cosmology
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astro-ph
|
The extreme luminosity and their fairly unique temporal behaviour have made
supernovae a superb tool to measure distances in the universe. As complex
astrophysical events they provide interesting insights into explosion physics,
explosive nucleosynthesis, hydrodynamics of the explosion and radiation
transport. They are an end product of stellar evolution and provide clues to
the stellar composition. Since they can be observed at large distances they
have become critical probes to further explore astrophysical effects, like dust
properties in external galaxies and the star formation history of galaxies.
Some of the astrophysics interferes with the cosmological applications of
supernovae. The local velocity field, distorted by the gravitational attraction
of the local large scale structure, and the reddening law appear at the moment
the major limitations in the accuracy with which cosmological parameters can be
determined. These absorption effects can introduce a secondary bias into the
observations of the distant supernovae, which needs to be carefully evaluated.
Supernovae have been used for the measurement of the Hubble constant, i.e. the
current expansion rate of the universe, and the accelerated cosmic expansion
directly inferred from the apparent faintness of the distant supernovae.
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No Label
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0706.3952
|
Arlin Crotts
|
Lunar Outgassing, Transient Phenomena and The Return to The Moon, II:
Predictions for Interactions between Outgassing and Regolith
|
astro-ph
|
We consider the implications from Paper I on how gas leaking through the
lunar surface might interact with the regolith, and in what respects this might
affect or cause the appearance of optical Transient Lunar Phenomena (TLPs). We
consider briefly a range of phenomena, but concentrate at the extremes of high
and low gas flow rate, which might represent the more likely behaviors.
Extremely fast i.e., explosive, expulsion of gas from the surface is
investigated by examining the minimal amount of gas needed to displace a plug
of regolith above a site of gaseous overpressure at the regolith's base. The
area and timescale of this disturbance, it is consistent with observed TLPs.
Furthermore there are several ways in which such an explosion might be expected
to change the lunar surface appearance in a way consistent with many TLPs,
including production of obscuration, brightening and color changes. At the slow
end of the volatile flow range, gas seeping from the interior is retained below
the surface for extensive times due to the low diffusivity of regolith
material. A special circumstance arises if the volatile flow contains water
vapor, because water is uniquely capable of freezing as it passes from the base
to the surface of the regolith. For a large TLP site, it is plausible to think
of areas on the square-km scale accumulating significant bodies of water ice.
Furthermore, as the system evolves over geological time, the ice accumulation
zone will evolve downwards into the regolith. Since many reactions possible
between the volatiles and regolith, depending on the additional gases besides
water, can act to decrease diffusivity in the regolith, it is plausible that
the volatiles produce a barrier between the seepage source and vacuum, forcing
the ice zone to expand to larger areas.
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No Label
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astro-ph/0407241
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Hideki Ozawa
|
The X-ray emission from Young Stellar Objects in the rho Ophiuchi cloud
core as seen by XMM-Newton
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astro-ph
|
We observed the main core F of the rho Ophiuchi cloud, an active star-forming
region located at ~140 pc, using XMM-Newton with an exposure of 33 ks. We
detect 87 X-ray sources within the 30' diameter field-of-view of the it EPIC
imaging detector array. We cross-correlate the positions of XMM-Newton X-ray
sources with previous X-ray and infrared (IR) catalogs: 25 previously unknown
X-ray sources are found from our observation; 43 X-ray sources are detected by
both XMM-Newton and Chandra; 68 XMM-Newton X-ray sources have 2MASS near-IR
counterparts. We show that XMM-Newton and Chandra have comparable sensitivity
for point source detection when the exposure time is set to ~30 ks for both. We
detect X-ray emission from 7 Class I sources, 26 Class II sources, and 17 Class
III sources. The X-ray detection rate of Class I sources is very high (64 %),
which is consistent with previous Chandra observations in this area. We propose
that 15 X-ray sources are new class III candidates, which doubles the number of
known Class III sources, and helps to complete the census of YSOs in this area.
We also detect X-ray emission from two young bona fide brown dwarfs, GY310 and
GY141, out of three known in the field of view. GY141 appears brighter by
nearly two orders of magnitude than in the Chandra observation. We extract
X-ray light curves and spectra from these YSOs, and find some of them showed
weak X-ray flares. We observed an X-ray flare from the bona fide brown dwarf
GY310. We find as in the previous Chandra observation of this region that Class
I sources tend to have higher temperatures and heavier X-ray absorptions than
Class II and III sources.
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astro-ph/9903026
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Joseph D. Lykken
|
New and Improved Superstring Phenomenology
|
astro-ph
|
Recent developments in string theory have important implications for
cosmology. Topics discussed here are inflation, the cosmological constant,
smoothing of cosmological singularities, and dark matter from parallel
universes. Talk presented at the International Workshop on Particle Physics and
the Early Universe (COSMO-98), 15-20 Nov, Asilomar, Monterey, CA.
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No Label
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astro-ph/0308518
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Aaron J. Romanowsky
|
A Dearth of Dark Matter in Ordinary Elliptical Galaxies
|
astro-ph
|
The kinematics of the outer parts of three intermediate-luminosity elliptical
galaxies have been studied using the Planetary Nebula Spectrograph. The
galaxies' velocity dispersion profiles are found to decline with radius;
dynamical modeling of the data indicates the presence of little if any dark
matter in these galaxies' halos. This surprising result conflicts with findings
in other galaxy types, and poses a challenge to current galaxy formation
theories.
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No Label
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astro-ph/0603300
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Dxwang
|
A Toy Model for Magnetic Connection in Black-Hole Accretion Disc
|
astro-ph
|
A toy model for magnetic connection in black hole (BH) accretion disc is
discussed based on a poloidal magnetic field generated by a single electric
current flowing around a Kerr black hole in the equatorial plane. We discuss
the effects of the coexistence of two kinds of magnetic connection (MC) arising
respectively from (1) the closed field lines connecting the BH horizon with the
disc (henceforth MCHD), and (2) the closed field lines connecting the plunging
region with the disc (henceforth MCPD). The magnetic field configuration is
constrained by conservation of magnetic flux and a criterion of the screw
instability of the magnetic field. Two parameters and are introduced to
describe our model instead of resolving the complicated MHD equations. Compared
with MCHD, energy and angular momentum of the plunging particles are extracted
via MCPD more effectively, provided that the BH spin is not very high. It turns
out that negative energy can be delivered to the BH by the plunging particles
without violating the second law of BH thermodynamics, however it cannot be
realized via MCPD in a stable way.
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astro-ph/9703090
|
James G. Bartlett
|
Galaxy Clusters in Cosmology: Cluster Abundance as a Probe of Structure
Formation
|
astro-ph
|
In gaussian theories of structure formation, the galaxy cluster abundance is
an extremely sensitive probe of the density fluctuation power spectrum and of
the density parameter, $\Omega$. We develop this theme by deriving and studying
in detail the mass function of collapsed objects and its relation to these
quantities. Application to current data yields constraints which are degenerate
between the amplitude of the perturbations and the density parameter; we
nevertheless obtain an important limit on the present day mass perturbation
amplitude as a function of $\Omega$ and can rule-out the `standard' cold dark
matter (CDM) model. Future observations of the evolution of the cluster
abundance will break the degeneracy and provide important constraints on both
the power spectrum and the density parameter, individually. We focus primarily
on X-ray clusters in the discussion, and finish with a presentation of the
promising new field of Sunyaev-Zel'dovich observations of cluster evolution.
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astro-ph/0005495
|
Michael R. Corbin
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A Color Analysis of the NICMOS Parallel Image Archive
|
astro-ph
|
We present a photometric analysis of all high Galactic lattitude (|b| > 20d)
broad-band parallel images taken by the Near Infrared Camera and Multi-Object
Spectrometer (NICMOS) instrument during its initial lifetime in HST Cycle 7.
These images, taken through the F110W and F160W filters, reach a mean 3-sigma
limiting magnitude of approximately 22 in both bands, and cover a total area of
approximately 92 square arcminutes. The reddest of the 358 galaxies detected
have F110W-F160W colors and F160W magnitudes consistent with being a
combination of both dusty star-forming and evolved early-type galaxies at 1 < z
< 2. The surface density of these galaxies is comparable to that of the
population of Extremely Red Objects (EROs) discovered in ground-based surveys
(~ 100 deg-2), suggesting that EROs also represent a combination of both galaxy
types in this redshift range. Roughly 10% of the detected galaxies appear to be
blue compact dwarf galaxies at z < 1, a result consistent with studies of the
HST Medium Deep Survey Fields. The surface density of these objects down to
magnitude of 22 in F160W is ~ 300 deg-2. None of the 681 point sources detected
have F110W-F160W colors matching those expected for QSOs whose continua have
been significantly reddened by internal dust. Our data limit the surface
density of such QSOs to be < 50 deg-2 down the mean limiting magnitudes of the
sample images. Since the surface density of QSOs detected on the basis of
ultraviolet excess in optical surveys to comparable depth is ~ 100 deg-2, this
argues against the suggestion that dust-reddened QSOs comprise the undetected
majority of the QSO population. The F110W-F160W color can also be used to
identify unreddened QSOs at z > 8, but we find no such candidates. This is
consistent with the evidence that QSO space density declines sharply at z > 5.
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astro-ph/9901016
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Raul Jimenez
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The role of star formation in the Tully-Fisher law
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astro-ph
|
We investigate the influence of the star formation rate on the Tully-Fisher
relation. We find that a simple model which combines the empirically-determined
star-formation rate with the expected properties of galaxy halos provides a
remarkably good fit to the absolute magnitude-rotation speed correlation. We
find that the power-law nature, its slope, normalisation and scatter, are all
readily accounted for if the Universe has a low density parameter, with or
without a cosmological constant and disks are assembled at $z \sim 1 - 1.5$.
Moreover, this agreement is found simultaneously in 4 wavebands. An Einstein-de
Sitter Universe produces disks which are too faint unless the disks are
assembled at $z\sim 0.5$. The scatter in the relation is due to a combination
of the expected range of spin parameters of the halos and the range of
formation redshifts. The source of the scatter opens up possibilities of a
better galaxy distance indicator, if spectroscopic observations of globular
clusters can be used to determine the halo rotation.
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No Label
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astro-ph/9901025
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Bruce Elmegreen
|
Galactic bulge formation as a maximum intensity starburst
|
astro-ph
|
Properties of normal galactic star formation, including the density
dependence, threshold density, turbulent scaling relations, and clustering
properties, are applied to the formation of galactic bulges. One important
difference is that the bulge potential well is too deep to have allowed
self-regulation or blow-out by the pressures from young stars, unlike galactic
disks or dwarf galaxies. As a result, bulge formation should have been at the
maximum rate, which is such that most of the gas would get converted into stars
in only a few dynamical time scales, or ~10^8 years. The gas accretion phase
can be longer than this, but once the critical density is reached, which
depends primarily on the total virial density from dark matter, the formation
of stars in the bulge should have been extremely rapid. Such three-dimensional
star formation should also have formed many clusters, like normal disk star
formation today. Some of these clusters may have survived as old globulars, but
most got dispersed, although they might still be observable as concentrated
streams in phase space.
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No Label
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astro-ph/0505127
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Todd Boroson
|
Blueshifted [O III] Emission: Indications of a Dynamic NLR
|
astro-ph
|
The [O III] 5007 line is commonly used as an indicator of the systemic
redshift of AGNs. Also, recent studies have used the width of this emission
line as a proxy for the stellar velocity dispersion in the host galaxy. This
paper calls both of these assumptions into question by analyzing a sample of
approximately 400 AGN spectra from the first data release of the Sloan Digital
Sky Survey. These spectra show that the low-ionization forbidden lines ([O II],
[N II], [S II]) define a consistent redshift, but that the peak of the [O III]
line is blueshifted in approximately half of the AGNs with respect to that
redshift. For the sample studied here, the average shift is 40 km/s, with the
largest shift being over 400 km/s. The magnitude of this shift is found to be
correlated with a number of properties, including the width of the [O III] line
and the Eddington ratio (L/L$_{Edd}$), derived from the luminosity and width of
H-beta.
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astro-ph/0205157
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Scott J. Kenyon
|
The Eclipsing Binary BG Geminorum: Improved Constraints on the Orbit and
the Structure of the Accretion Disk
|
astro-ph
|
We describe new optical photometric and spectroscopic observations of the
semi-detached eclipsing binary BG Geminorum. A large change in the amount of Mg
I absorption at secondary maximum indicates the presence of cool material in
the outer edge of the disk surrounding the unseen primary star. Detection of
weak He I emission implies a hot radiation source at the inner edge of the
disk. If the velocity variations in the Hbeta emission line track the orbital
motion of the primary star, the primary star has an orbital semiamplitude of K1
= K(Hbeta) = 16.0 +- 4.6 km/sec. This result yields a mass ratio, q = 0.22 +-
0.07, consistent with the q = 0.1 derived from the large ellipsoidal variation.
Despite this progress, the nature of the primary star - B-type star or black
hole - remains uncertain.
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astro-ph/0406312
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Shumei Jia
|
The analysis of Abell 1835 using a deprojection technique
|
astro-ph
|
We present the results from a detailed deprojection analysis of Abell 1835 as
observed by {\it XMM-Newton}. If we fit the spectra with an isothermal plasma
model, the deprojected temperature profile is flat in the outer region around
7.6 keV and decreases to $\sim$ 5.6 keV in the center, which may be connected
with the gas cooling. In the central part, a two-component thermal plasma model
can fit the spectrum significantly better. Moreover, the cool component (T
$\sim$ 1.8 keV) has a much lower metal abundance than the hot component (T
$\sim$ 8 keV), which may be due to the longer cooling time for the cool gas
with lower abundance. In addition, it was found that without a main isothermal
component, the standard cooling flow model cannot fit the spectrum
satisfactorily. From the isothermal model fitting results we also derived the
electron density $n_e$, and fitted its radial distribution with a
double-$\beta$ model. The $n_e$ profile inferred with the double-$\beta$ model
and the deprojected X-ray gas temperature profile were then combined to derive
the total mass and the total projected mass of the cluster. The projected mass
is lower than that derived from the weak lensing method. However, assuming that
the cluster extends to a larger radius $\sim15'$ as found by Clowe & Schneider
(2002), the two results are consistent within the error bars. Furthermore, we
calculated the projected mass within the radius of $\sim$ 153 kpc implied by
the presence of a gravitational lensing arc, which is about half of the mass
determined from the optical lensing.
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No Label
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astro-ph/0109025
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Vladimir N. Lukash
|
Cosmological implications of galaxy clusters: best-fit models
|
astro-ph
|
The galaxy cluster power spectrum and mass/temperature functions are
currently the most precise observational tools for constraining the theory of
the formation of large scale structure (LSS) in the Universe. Complementing
these tests by the observational data at larger (cosmic microwave backgroud
anisortopy (CMBA)) and smaller (distribution of $Ly_\alpha$ clouds) scales
opens the way to a straightforward determination of the cosmological parameters
in simplest dark matter (DM) models. We argue that such a 'minimal data set' is
free from systematic effects and can indicate quite precisely the parameters of
spatially flat mixed DM model with a positive cosmological constant and no
cosmic gravitational waves.
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No Label
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astro-ph/0606652
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Andrew Clarke
|
Evidence for variable outflows in the Young Stellar Object V645 Cygni
|
astro-ph
|
As part of the Red MSX Source Survey of Massive Young Stellar Objects (MYSOs)
we have conducted multi-wavelength follow up observations of the well-known
object V645 Cygni. We present our data on this object, whose near-infrared
spectrum is exceptional and place these in context with previous observations.
Our observations of V645 Cyg included near/mid infrared imaging observations,
13CO 2-1 line observations and high signal-to-noise velocity resolved
near-infrared spectroscopy. The spectrum shows P-Cygni hydrogen Brackett
emission, consistent with a high velocity stellar wind. A red-shifted emission
component to a number of near-IR emission lines was also uncovered. This is
associated with a similar component in the H alpha line. V645 Cyg is also found
to have variable CO first overtone bandhead emission. The data clearly indicate
that the outflow of V645 Cyg is variable. The unidentified feature in a
previously published optical spectrum is identified with a receding outflow at
2000 km per second. The nature of this feature, which is found in hydrogen and
helium atomic lines and CO molecular lines remains a puzzle.
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astro-ph/0108142
|
Sandhya Rao
|
A Comparative Study of Damped Lyman Alpha Galaxies
|
astro-ph
|
We compare the properties of a sample of local (z=0) gas-rich galaxies
studied in 21 cm emission to a sample of 13 low-redshift (z < 1) damped Lyman
alpha (DLA) galaxies identified as the counterparts of low-redshift DLA systems
found in QSO absorption-line surveys. This absorption-selected sample has
average redshift <z> = 0.5. We find that many of the properties of the two
samples are comparable. However, consideration of the statistical results on
all known low-redshift DLA systems indicates that there is: (1) a somewhat
higher incidence and cosmological mass density for the low-redshift DLA systems
in comparison to expectations at z=0 and (2) an unexpectedly high rate of
occurrence of very large column density [N(HI) > 1E21 atoms/cm^2] low-redshift
DLA systems; both of these results are discussed by Rao & Turnshek in these
proceedings. These differences, coupled with imaging studies, suggest that
there may be an excess of low-redshift DLA galaxies in the form of dwarf and/or
low surface brightness galaxies. Some examples of low-redshift DLA galaxies are
shown by Nestor et al. in these proceedings.
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No Label
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0804.3712
|
Gustavo Porto de Mello
|
The Alpha Centauri Binary System: Atmospheric Parameters and Element
Abundances
|
astro-ph
|
The alpha Centauri binary system, owing to its duplicity, proximity and
brightness, and its components' likeness to the Sun, is a fundamental
calibrating object for the theory of stellar structure and evolution and the
determination of stellar atmospheric parameters. This role, however, is
hindered by a considerable disagreement in the published analyses of its
atmospheric parameters and abundances. We report a new spectroscopic analysis
of both components of the alpha Centauri binary system and compare published
analyses of the system. The analysis is differential with respect to the Sun,
based on high-quality spectra, and employed spectroscopic and photometric
methods to obtain as many independent Teff determinations as possible. The
atmospheric parameters are also checked for consistency against the results of
the dynamical analysis and the positions of the components in a theoretical HR
diagram. We discuss possible origins of discrepancies, concluding that the
presence of NLTE effects is a probable candidate, but we note that there is as
yet no consensus on the existence and cause of an offset between the
spectroscopic and photometric Teff scales of cool dwarfs. The spectroscopic
surface gravities also agree with those derived from directly measured masses
and radii. The abundance pattern can be deemed normal in the context of recent
data on metal-rich stars. The position of alpha Cen A in an up-to-date
theoretical evolutionary diagrams yields a good match of the evolutionary mass
and age with those from the dynamical solution and seismology.
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astro-ph/0302393
|
John S. Mulchaey
|
An X-ray Atlas of Groups of Galaxies
|
astro-ph
|
A search was conducted for a hot intragroup medium in 109 low-redshift galaxy
groups observed with the ROSAT PSPC. Evidence for diffuse, extended X-ray
emission is found in at least 61 groups. Approximately one-third of these
detections have not been previously reported in the literature.
Most of the groups are detected out to less than half of the virial radius
with ROSAT. Although some spiral-rich groups do contain an intragroup medium,
diffuse emission is restricted to groups that contain at least one early-type
galaxy.
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No Label
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0705.0878
|
Ralf-Dieter Scholz
|
Astrophysical supplements to the ASCC-2.5. Ia. Radial velocities of
about 55000 stars and mean radial velocities of 516 Galactic open clusters
and associations
|
astro-ph
|
We present the 2nd version of the Catalogue of Radial Velocities with
Astrometric Data (CRVAD-2). This is the result of the cross-identification of
stars from the All-Sky Compiled Catalogue of 2.5 Million Stars (ASCC-2.5) with
the General Catalogue of Radial Velocities and with other recently published
radial velocity lists and catalogues. The CRVAD-2 includes accurate J2000
equatorial coordinates, proper motions and trigonometric parallaxes in the
Hipparcos system, $B, V$ photometry in the Johnson system, spectral types,
radial velocities (RVs), multiplicity and variability flags for 54907 ASCC-2.5
stars. We have used the CRVAD-2 for a new determination of mean RVs of 363 open
clusters and stellar associations considering their established members from
proper motions and photometry in the ASCC-2.5. For 330 clusters and
associations we compiled previously published mean RVs from the literature,
critically reviewed and partly revised them. The resulting Catalogue of Radial
Velocities of Open Clusters and Associations (CRVOCA) contains about 460 open
clusters and about 60 stellar associations in the Solar neighbourhood. These
numbers still represent less than 30% of the total number of about 1820 objects
currently known in the Galaxy. The mean RVs of young clusters are generally
better known than those of older ones.
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astro-ph/0701018
|
A. J. Barth
|
A Normal Stellar Disk in the Galaxy Malin 1
|
astro-ph
|
Since its discovery, Malin 1 has been considered the prototype and most
extreme example of the class of giant low surface brightness disk galaxies.
Examination of an archival Hubble Space Telescope I-band image reveals that
Malin 1 contains a normal stellar disk that was not previously recognized,
having a central I-band surface brightness of mu_0 = 20.1 mag arcsec^-2 and a
scale length of 4.8 kpc. Out to a radius of ~10 kpc, the structure of Malin 1
is that of a typical SB0/a galaxy. The remarkably extended, faint outer
structure detected out to r~100 kpc appears to be a photometrically distinct
component and not a simple extension of the inner disk. In terms of its disk
scale length and central surface brightness, Malin 1 was originally found to be
a very remote outlier relative to all other known disk galaxies. The presence
of a disk of normal size and surface brightness in Malin 1 suggests that such
extreme outliers in disk properties probably do not exist, but underscores the
importance of the extended outer disk regions for a full understanding of the
structure and formation of spiral galaxies.
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astro-ph/0406467
|
Asantha R. Cooray
|
Gravitational Wave Background of Neutron Star-White Dwarf Binaries
|
astro-ph
|
We discuss the stochastic background of gravitational waves from ultra
compact neutron star-white dwarf (NS-WD) binaries at cosmological distances.
Under the assumption that accreting neutron stars and donor white dwarf stars
form most of the low mass X-ray binaries (LMXBs), our calculation makes use of
recent results related to the luminosity function determined from X-ray
observations. Even after accounting for detached NS-WD binaries not captured in
X-ray data, the NS-WD background is at least an order of magnitude below that
due to extragalactic white dwarf-white dwarf binaries and below the
detectability level of the Laser Interferometer Space Antenna (LISA) at
frequencies between 10^-5 Hz and 10^-1 Hz. While the extragalactic background
is unlikely to be detected, we suggest that around one to ten galactic NS-WD
binaries may be resolved with LISA such that their positions are determined to
an accuracy of several degrees on the sky.
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astro-ph/9911414
|
Greg Taylor
|
Magnetic Fields in Quasar Cores II
|
astro-ph
|
Multi-frequency polarimetry with the Very Long Baseline Array (VLBA)
telescope has revealed absolute Faraday Rotation Measures (RMs) in excess of
1000 rad/m/m in the central regions of 7 out of 8 strong quasars studied (e.g.,
3C 273, 3C 279, 3C 395). Beyond a projected distance of ~20 pc, however, the
jets are found to have |RM| < 100 rad/m/m. Such sharp RM gradients cannot be
produced by cluster or galactic-scale magnetic fields, but rather must be the
result of magnetic fields organized over the central 1-100 pc. The RMs of the
sources studied to date and the polarization properties of BL Lacs, quasars and
galaxies are shown to be consistent so far with the predictions of unified
schemes. The direct detection of high RMs in these quasar cores can explain the
low fractional core polarizations usually observed in quasars at centimeter
wavelengths as the result of irregularities in the Faraday screen on scales
smaller than the telescope beam. Variability in the RM of the core is reported
for 3C 279 between observations taken 1.5 years apart, indicating that the
Faraday screen changes on that timescale, or that the projected superluminal
motion of the inner jet components samples a new location in the screen with
time. Either way, these changes in the Faraday screen may explain the dramatic
variability in core polarization properties displayed by quasars.
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astro-ph/0608032
|
Steven Furlanetto
|
Cosmology at Low Frequencies: The 21 cm Transition and the High-Redshift
Universe
|
astro-ph
|
Observations of the high-redshift Universe with the 21 cm hyperfine line of
neutral hydrogen promise to open an entirely new window onto the early phases
of cosmic structure formation. Here we review the physics of the 21 cm
transition, focusing on processes relevant at high redshifts, and describe the
insights to be gained from such observations. These include measuring the
matter power spectrum at z~50, observing the formation of the cosmic web and
the first luminous sources, and mapping the reionization of the intergalactic
medium. The epoch of reionization is of particular interest, because large HII
regions will seed substantial fluctuations in the 21 cm background. We also
discuss the experimental challenges involved in detecting this signal, with an
emphasis on the Galactic and extragalactic foregrounds. These increase rapidly
toward low frequencies and are especially severe for the highest redshift
applications. Assuming that these difficulties can be overcome, the redshifted
21 cm line will offer unique insight into the high-redshift Universe,
complementing other probes but providing the only direct, three-dimensional
view of structure formation from z~200 to z~6.
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astro-ph/0211103
|
Andrea De Luca
|
HST proper motion confirms the optical detection of PSR B1929+10
|
astro-ph
|
We have measured the proper motion of the candidate optical counterpart of
the old, nearby pulsar PSR B1929+10, using a set of HST/STIS images collected
in 2001, 7.2 years after the epoch of the original FOC detection (Pavlov et al.
1996). The yearly displacement, mu=107.3+/-1 mas/yr along a position angle of
64.6+/-0.6 deg, is fully consistent with the most recent VLBA radio
measurement. This result provides a robust confirmation of the identification
of PSR B1929+10 in the optical band.
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0812.3972
|
Micha{\l} Chodorowski
|
Eppur si muove
|
astro-ph
|
In two recent papers, Abramowicz et al. claim that the expansion of the
Universe can be interpreted only as the expansion of space. In fact, what they
really prove is that the cosmological expansion cannot be described in terms of
real motions in Minkowski spacetime. However, there is no controversy about
this issue. Abramowicz et al. show that in general, the cosmological redshift
is not a Doppler shift and they consider this fact as a proof that space
expands. Again, nobody believes (perhaps except Milne) that for non-empty
universes the origin of the redshift is purely Dopplerian. From the Principle
of Equivalence it follows that there must be also a gravitational shift in
presence of matter. Indeed, it is well known in cosmology that for small
redshifts, the cosmological redshift can be decomposed into a Doppler component
and a gravitational component. In a forthcoming paper, we shall perform such a
decomposition for arbitrarily large values of the redshift.
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astro-ph/0309184
|
Kirsten Kraiberg Knudsen
|
A new deep SCUBA survey of gravitationally lensing clusters
|
astro-ph
|
We have conducted a new deep SCUBA survey, which has targetted 12 lensing
galaxy clusters and one blank field. In this survey we have detected several
sub-mJy sources after correcting for the gravitational lensing by the
intervening clusters. We here present the preliminary results and point out two
highlights.
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No Label
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astro-ph/0703526
|
Ugo Becciani
|
FLY: MPI-2 High Resolution code for LSS Cosmological Simulations
|
astro-ph
|
Cosmological simulations of structures and galaxies formations have played a
fundamental role in the study of the origin, formation and evolution of the
Universe. These studies improved enormously with the use of supercomputers and
parallel systems and, recently, grid based systems and Linux clusters. Now we
present the new version of the tree N-body parallel code FLY that runs on a PC
Linux Cluster using the one side communication paradigm MPI-2 and we show the
performances obtained. FLY is included in the Computer Physics Communication
Program Library. This new version was developed using the Linux Cluster of
CINECA, an IBM Cluster with 1024 Intel Xeon Pentium IV 3.0 Ghz. The results
show that it is possible to run a 64 Million particle simulation in less than
15 minutes for each timestep, and the code scalability with the number of
processors is achieved. This lead us to propose FLY as a code to run very large
N-Body simulations with more than $10^{9}$ particles with the higher resolution
of a pure tree code. The FLY new version will be available at
http://www.ct.astro.it/fly/ and CPC Program Library.
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astro-ph/0702416
|
Marie-No\"elle C\'el\'erier
|
The Accelerated Expansion of the Universe Challenged by an Effect of the
Inhomogeneities. A Review
|
astro-ph
|
Since its decovery during the late 90's, the dimming of distant SN Ia
apparent luminosity has been mostly ascribed to the influence of a mysterious
dark energy component. Formulated in a Friedmannian cosmological modelling
framework based upon the cosmological ``principle'' hypothesis, this
interpretation has given rise to the ``concordance'' model. However, a caveat
of such a reasoning is that the cosmological ``principle'' derives from a
philosophical Copernican assumption and has never been tested. Furthermore, a
weakness of its conclusion, i. e., the existence of a negative-pressure fluid
or a cosmological constant, is that it would have profound implications for the
current theories of physics. This is why we have proposed a more conservative
explanation, ascribing the departure of the observed universe from an
Einstein-de Sitter model to the influence of inhomogeneities. This idea has
been independently developed by other authors and further enlarged to the
reproduction of different cosmological data. We review here the main proposals
which have been put forward to deal with this purpose and present some
prospects for future developments.
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astro-ph/0007358
|
William B. Sparks
|
Face-on Dust Disks in Galaxies with Optical Jets
|
astro-ph
|
The presence of optical synchrotron jets in radio galaxies is relatively
rare. Here, we show that of the nearest five FR-I 3CR radio galaxies showing
optical jets, {\it four} show evidence for almost circular, presumably face-on,
dust disks. This is strong support for the two-fold idea that (\romannumeral1)
jets emerge close to perpendicular to inner gas disks and (\romannumeral2)
optical non-thermal synchrotron emission is seen only when the jet points
towards the observer. The implied critical angle to the line-of-sight is
approximately 30 - $40^{\circ}$, i.e. if the angle of the jet to the
line-of-sight is less than about $40^{\circ}$ we see an optical jet. The
corresponding relativisitic $\gamma$ factor is $\approx 1.5$ which is
consistent with current observations of jet proper motion that show a range up
to $\gamma \sim 6$ for M87. The relatively low speeds implied by $\gamma
\approx 1.5$ may be due to a global deceleration of the jet as in unified
theories, or else to stratification within the jet. Unresolved nuclei are
common in the optical. Their luminosities are also consistent with the beaming
concept when compared to inclination inferred from the dust lanes. The disk
sizes are typically several hundred parsecs, to kiloparsec size. The galaxy
with an optical jet that does not show a face-on disk, M87, instead has more
complex radial dust and ionized gas filaments.
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astro-ph/0601329
|
Arnon Dar
|
The origin of cosmic rays at all energies
|
astro-ph
|
There is mounting evidence from observations of long duration gamma ray
bursts (GRBs), supernova remnants (SNR) and the supernova (SN) explosion 1987A,
that SN explosions eject highly relativistic bipolar jets of plasmoids
(cannonballs) of ordinary matter. Here we use the remarkably successful
cannonball (CB) model of GRBs to show that bipolar jets from Galactic SN
explosions can produce the bulk of the Galactic cosmic rays at energies below
the ankle, while the CRs which escape into the intergalactic space or are
deposited there directly by jets from SNe in external galaxies can produce the
observed cosmic ray flux with energies above the ankle. The model predict well
all the observed properties of cosmic rays: their intensity, their spectrum
including their elemental knees and ankle, their composition and the
distribution of their arrival directions. At energies above the CR ankle, the
Galactic magnetic fields can no longer delay the free escape of such ultra high
energy CRs (UHECRs) from the Galaxy. The UHECRs, which are injected into the
intergalactic medium (IGM) by the SN jets from our Galaxy and all the other
galaxies and are isotropized there by the IGM magnetic fields, dominate the
flux of UHECRs. Almost all the extragalactic UHECRs heavier than helium
photo-disintegrate in collisions with the far infrared (FIR), microwave and
radio background radiations. The CR protons and He nuclei however suffer a
Greisen-Zatsepin-Kuzmin (GZK) cutoff due to pion photo-production in collisions
with the FIR, microwave and radio background photons.
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astro-ph/0302593
|
Allan Sacha Brun
|
Solar Turbulence and Magnetism Studied Within a Rotating Convective
Spherical Shell
|
astro-ph
|
We discuss recent advances made in modelling the complex magnetohydrodynamics
of the Sun using our anelastic spherical harmonics (ASH) code. We have
conducted extensive 3--D simulations of compressible convection in rotating
spherical shells with and without magnetic fields, to study the coupling
between global-scale convection and rotation in seeking to understand how the
solar differential rotation is established and maintained. Such simulations
capable of studying fairly turbulent convection have been enabled by massively
parallel supercomputers. The resulting convection within domains that capture a
good fraction of the bulk of the convection zone is highly time dependent and
intricate, and is dominated by intermittent upflows and networks of strong
downflows. A high degree of coherent structures involving downflowing plumes
can be embedded in otherwise chaotic flow fields. These vortical structures
play a significant role in yielding Reynolds stresses that serve to
redistribute angular momentum, leading to differential rotation profiles with
pole to equator contrasts of about 30% in angular velocity $\Omega$ and some
constancy along radial line at mid latitudes, thereby making good contact with
deductions from helioseismology. When a magnetic field is introduced, a dynamo
regime can be found that does not destroy the strong differential rotation
achieved in pure hydrodynamics cases. The magnetic fields are found to
concentrate around the downflowing networks and to have significant north-south
asymmetry and helicity.
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astro-ph/0701757
|
Mercedes Molla
|
Evolutionary self-consistent models of HII galaxies
|
astro-ph
|
We have studied the viability of new theoretical models which combine a
chemical evolution code, an evolutionary synthesis code and a photoionization
code, to understand the star formation and evolution of H{\sc ii} galaxies. The
emission lines observed in H{\sc ii} galaxies are reproduced by meas of the
photoionization code CLOUDY, using as ionizing spectrum the spectral energy
distribution of the modeled H{\sc ii} galaxy, which, in turn, is calculated
according to a Star Formation History (SFH) and a metallicity evolution given
by a chemical evolution model. Our technique reproduces the observed diagnostic
diagrams and equivalent width-color correlations for local H{\sc ii} galaxies.
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astro-ph/0308015
|
L. F. Olsen
|
Multi-object spectroscopy of low redshift EIS clusters. II
|
astro-ph
|
We present the results of carrying out multi-object spectroscopy in 10 EIS
cluster fields. Based on the list of 345 galaxy redshifts we identify
significant 3D-density enhancements. For 9 of the EIS clusters we identify
significant 3D-concentrations corresponding to the originally detected cluster
candidate. We find redshifts in the range 0.097<=z<=0.257 which is in good
agreement with the matched filter estimate of z_MF=0.2. We estimate velocity
dispersions in the range 219-1160 km/s for the confirmed clusters.
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astro-ph/0608098
|
Rajagopalan Ramachandran
|
Intrinsic short time scale variability of W3(OH) maser
|
astro-ph
|
We have studied the OH masers in the star forming region, W3(OH), with data
obtained from the Very Long Baseline Array (VLBA). The data provide an angular
resolution of $\sim$5 mas, and a velocity resolution of 106 m s$^{-1}$. A novel
analysis procedure allows us to differentiate between broadband temporal
intensity fluctuations introduced by instrumental gain variations plus
interstellar diffractive scintillation, and intrinsic narrowband variations.
Based on this 12.5 hours observation, we are sensitive to variations with time
scales of minutes to hours. We find statistically significant intrinsic
variations with time scales of $\sim$15--20 minutes or slower, based on the
{\it velocity-resolved fluctuation spectra}. These variations are seen
predominantly towards the line shoulders. The peak of the line profile shows
little variation, suggesting that they perhaps exhibit saturated emission. The
associated modulation index of the observed fluctuation varies from
statistically insignificant values at the line center to about unity away from
the line center. Based on light-travel-time considerations, the 20-minute time
scale of intrinsic fluctuations translates to a spatial dimension of $\sim$2--3
AU along the sight-lines. On the other hand, the transverse dimension of the
sources, estimated from their observed angular sizes of about $\sim$3 mas, is
about 6 AU. We argue that these source sizes are intrinsic, and are not
affected by interstellar scatter broadening. The implied peak brightness
temperature of the 1612/1720 maser sources is about $\sim2\times 10^{13}$ K,
and a factor of about five higher for the 1665 line.
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astro-ph/0201076
|
Shin Sasaki
|
Scaling Relation to Understand Non-Detection of Cold Gas at the Cluster
Center
|
astro-ph
|
Recent XMM-Newton observations of clusters of galaxies have indicated the
soft X-ray spectra to be inconsistent with the simple isobaric cooling flow
model. There is almost no feature of the cold gas expected from the model. This
shows that we have not yet understood the physics of the hot gas in clusters of
galaxies well. A quantitative evaluation of the behavior of gas cooling is
important not only for understanding the clusters, themselves, but also for
studying cosmology and galaxy formation. To clarify the problem of this
reported discrepancy, we have studied scaling relations for clusters of
galaxies based on the self-similarity assumption. We also propose an
observational strategy to solve this problem.
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astro-ph/0111399
|
Martino Romaniello
|
Accurate Stellar Population Studies from Multiband Photometric
Observations
|
astro-ph
|
We present a new technique based on multi-band near ultraviolet and optical
photometry to measure both the stellar intrinsic properties, ie luminosity and
effective temperature, and the interstellar dust extinction along the line of
sight to hundreds of stars per square arcminute. The yield is twofold. On the
one hand, the resulting reddening map has a very high spatial resolution, of
the order of a few arcseconds, and can be quite effectively used in regions
where the interstellar material is patchy, thus producing considerable
differential extinction on small angular scales. On the other hand, combining
the photometric information over a wide baseline in wavelength provides an
accurate determination of temperature and luminosity for thousands of stars. As
a test case, we present the results for the region around Supernova 1987A in
the Large Magellanic Cloud imaged with the WFPC2 on board the Hubble Space
Telescope.
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0707.1547
|
Jules P. Halpern
|
The Next Geminga: Search for Radio and X-ray Pulsations from the Neutron
Star Identified with 3EG J1835+5918
|
astro-ph
|
We report unsuccessful searches for pulsations from the neutron star RX
J1836.2+5925 identified with the EGRET source 3EG J1835+5918. A 24 hr
observation with the NRAO Green Bank Telescope at 820 MHz placed an upper limit
on flux density of 17 uJy for P > 10 ms, and gradually increasing limits for 1
< P < 10 ms. The equivalent luminosity is lower than that of any known pulsar
with the possible exception of the radio-quiet gamma-ray pulsar Geminga. A set
of observations with the Chandra X-ray Observatory HRC totaling 118 ks revealed
no pulsar with 1 ms < P < 10 s. The upper limit on its pulsed fraction is 35%
assuming a sinusoidal pulse shape. The position of RX J1836.2+5925 in Chandra
observations separated by 3 years is unchanged within errors, leading to an
upper limit on its proper motion of <0.14"/yr, or v < 530 km/s at d = 800 pc, a
maximum distance estimated from its thermal X-ray spectrum. With these null
results, the properties of 3EG J1835+5918 and its X-ray counterpart RX
J1836.2+5925 are consistent with a more distant or older version of Geminga, or
perhaps a recycled pulsar. Having nearly exhausted the capabilities of current
instrumentation at all wavelengths, it will likely fall to the Gamma-ray Large
Area Space Telescope to discover pulsations from 3EG J1835+5918.
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astro-ph/9703006
|
Lipunova Galina
|
A Burst of Electromagnetic Radiation from a Collapsing Magnetized Star
|
astro-ph
|
The pattern of variations in the intensity of magnetodipole losses is studied
with the relativistic effect of magnetic-field dissipation during collapse into
a black hole taken into account.
A burst-type solution can be obtained both for a direct collapse and for the
formation of a rapidly-rotating, self-gravitating object - a spinar - using a
simple model. Analytical dependences on radius describing an electromagnetic
burst are derived. The time dependence of the burst shape for an infinitely
distant observer and the maximum energy of relativistic particles accelerated
by an electric field are numerically calculated.
The objects under consideration are of particular interest because particles
in their vicinity can be accelerated up to the Planck energies. Possible
astrophysical applications to the theory of active galactic nuclei (AGNs) and
QSOs are briefly discussed. It is shown for the first time that a spinar can be
produced by a merger of neutron stars; this possibility is considered in and
without connection with the formation of gamma-ray bursts.
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astro-ph/0510579
|
Adam J. Burgasser
|
Resolved Spectroscopy of M Dwarf/L Dwarf Binaries. I. DENIS
J220002.05-303832.9AB
|
astro-ph
|
We present the discovery of the common proper motion M9 + L0 binary DENIS
J220002.05-303832.9AB, identified serendipitously with the SpeX near infrared
imager/spectrograph. Spectral types are derived from resolved near infrared
spectroscopy of the well-separated (1"09+/-0"06) components and comparison to
equivalent data for M and L dwarf spectral standards. Physical association is
deduced from the angular proximity of the sources, their common proper motion
and their similar spectrophotometric distances (35+/-2 pc). The estimated
distance of this pair implies a projected separation of 38+/-3 AU, wider than
typical separations for other M dwarf/L dwarf binaries, but consistent with the
maximum separation/total system mass trend previously identified by Burgasser
et al. (2003). We discuss the DENIS 2200-3038AB system in context with other
low mass binaries, and its role in studying dust formation processes and
activity trends across the transition between the M and L dwarf spectral
classes.
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astro-ph/0011159
|
Mario Gliozzi
|
XMM-Newton monitoring of X-ray variability in the quasar PKS 0558-504
|
astro-ph
|
We present the temporal analysis of X-ray observations of the radio-loud
Narrow-Line Seyfert 1 galaxy (NLS1) PKS 0558-504 obtained during the XMM-Newton
Calibration and Performance Verification (Cal/PV) phase. The long term light
curve is characterized by persistent variability with a clear tendency for the
X-ray continuum to harden when the count rate increases. Another strong
correlation on long time scales has been found between the variability in the
hard band and the total flux. On shorter time scales the most relevant result
is the presence of smooth modulations, with characteristic time of ~ 2 hours
observed in each individual observation. The short term spectral variability
turns out to be rather complex but can be described by a well defined pattern
in the hardness ratio-count rate plane.
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astro-ph/0406571
|
Henry Lee
|
Chemical Abundances of H II Regions in the Starburst Galaxy NGC 1705
|
astro-ph
|
(Shortened version) We report optical spectroscopy of 16 H II regions in NGC
1705 and [O III] 4363 detections for the first time in five H II regions. The
resulting mean oxygen abundance derived directly from measured electron
temperatures is 12+log(O/H) = 8.21 +/- 0.05, which corresponds to [O/H] =
-0.45, or 35% of the solar value. There are no significant spatial
inhomogeneities in [O III] 4363 oxygen abundances from H II regions at a radius
approximately 10 arcsec from the super star cluster. In H II regions where [O
III] 4363 was not measured, oxygen abundances derived with bright-line methods
(accurate only to 0.2 dex) are in agreement with direct values of the oxygen
abundance. Faint narrow He II 4686 emission is found in two H II regions, but
the implied contribution from O^+3 to the total oxygen abundance is only 0.01
dex. Interestingly, the nitrogen-to-oxygen abundance ratio in the ionized H II
gas agrees with the value for the neutral H I, even though the metallicity of
the neutral gas may be a factor of six lower than that of the ionized gas. This
may be indicative of low-metallicity gas in the halo of the galaxy. Extinction
values, A_V, derived from observed Balmer line ratios along lines of sight to H
II regions are in the range between zero and 0.9 mag. With respect to the
metallicity-luminosity and metallicity-gas fraction diagnostics, the measured
oxygen abundance for NGC 1705 is comparable to Local Group dwarf irregulars at
a given luminosity and gas fraction.
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0803.0609
|
Lex Kaper
|
X-shooter: a medium-resolution, wide-band spectrograph for the VLT
|
astro-ph
|
X-shooter is the first second-generation instrument for the ESO Very Large
Telescope and will be installed in 2008. It is intended to become the most
powerful optical & near-infrared medium-resolution spectrograph in the world,
with a unique spectral coverage from 300 to 2500 nm in one shot. The X-shooter
consortium members are from Denmark, France, Italy, The Netherlands and ESO.
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astro-ph/0307256
|
Timothy M. Brown
|
Expected Detection and False Alarm Rates for Transiting Jovian Planets
|
astro-ph
|
Ground-based searches for transiting Jupiter-sized planets have so far
produced few detections of planets, but many of stellar systems with eclipse
depths, durations, and orbital periods that resemble those expected from
planets. I show that these detection rates are consistent with our present
knowledge of binary and multiple-star systems, and of Jovian-mass extrasolar
planets. Upcoming space-based searches for transiting Earth-sized planets will
be largely unaffected by the sources of false alarms that afflict current
ground-based searches, with one exception, namely distant eclipsing binaries
whose light is strongly diluted by that of a foreground star. A byproduct of
the rate estimation is evidence that the period distribution of extrasolar
planets is depressed for periods between 5 and 200 days.
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No Label
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No Label
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0710.1836
|
Jos\'e Pablo Fonfr\'ia Exp\'osito
|
High-J v=0 SiS Maser Emission in IRC+10216: A New Case of Infrared
Overlaps
|
astro-ph
|
We report on the first detection of maser emission in the J=11-10, J=14-13
and J=15-14 transitions of the v=0 vibrational state of SiS toward the C-rich
star IRC+10216. These masers seem to be produced in the very inhomogeneous
region between the star and the inner dust formation zone, placed at 5-7 R*,
with expansion velocities below 10 km/s. We interpret the pumping mechanism as
due to overlaps between v=1-0 ro-vibrational lines of SiS and mid-IR lines of
C2H2, HCN and their 13C isotopologues. The large number of overlaps found
suggests the existence of strong masers for high-J v=0 and v=1 SiS transitions,
located in the submillimeter range. In addition, it could be possible to find
several rotational lines of the SiS isotopologues displaying maser emission.
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astro-ph/0612629
|
Almudena Arcones
|
Neutrino-driven wind and wind termination shock in supernova cores
|
astro-ph
|
The neutrino-driven wind from a nascent neutron star at the center of a
supernova expands into the earlier ejecta of the explosion. Upon collision with
this slower matter the wind material is decelerated in a wind termination
shock. By means of hydrodynamic simulations in spherical symmetry we
demonstrate that this can lead to a large increase of the wind entropy,
density, and temperature, and to a strong deceleration of the wind expansion.
The consequences of this phenomenon for the possible r-process nucleosynthesis
in the late wind still need to be explored in detail. Two-dimensional models
show that the wind-ejecta collision is highly anisotropic and could lead to a
directional dependence of the nucleosynthesis even if the neutrino-driven wind
itself is spherically symmetric.
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0812.2181
|
Heidi Korhonen
|
Radial differential rotation vs surface differential rotation:
investigation based on dynamo models
|
astro-ph
|
Differential rotation plays a crucial role in the alpha-omega dynamo, and
thus also in creation of magnetic fields in stars with convective outer
envelopes. Still, measuring the radial differential rotation on stars is
impossible with the current techniques, and even the measurement of surface
differential rotation is difficult. In this work we investigate the surface
differential rotation obtained from dynamo models using similar techniques as
are used on observations, and compare the results with the known radial
differential rotation used when creating the Dynamo model.
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No Label
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astro-ph/0101360
|
Ledenev V. G.
|
On an Estimation of the Coronal Magnetic Field Strength from
Spectrographic Observations in the Microwave Range
|
astro-ph
|
Observations of the solar radio emission in the microwave frequency range
show fine structures consisting of a number of the almost parallel narrow
frequency bands. We interpret these bands as the cyclotron harmonics plasma
emission. This emission is generated by the anisotropic electron beam, which
excites longitudinal waves on the normal Doppler effect resonance. Then the
longitudinal waves convert into the radio emission on the second harmonic of
the longitudinal waves frequency and sometimes into the fundamental one.
Estimations of the magnetic field strength made on the basis of such a model
give the values of the magnetic field in the microwave burst sources as ~
100-200 G. The estimations of density variations are also given.
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astro-ph/0404556
|
Ra\'ul Sevilla
|
The structure of the ICM from High Resolution SPH simulations
|
astro-ph
|
We present results from a set of high (512^3 effective resolution), and
ultra-high (1024^3) SPH adiabatic cosmological simulations of cluster formation
aimed at studying the internal structure of the intracluster medium (ICM). We
derive a self-consistent analytical model of the structure of the intracluster
medium (ICM). We discuss the radial structure and scaling relations expected
from purely gravitational collapse, and show that the choice of a particular
halo model can have important consequences on the interpretation of
observational data. The validity of the approximations of hydrostatic
equilibrium and a polytropic equation of state are checked against results of
our simulations. The properties of the ICM are fully specified when a
'universal' profile is assumed for either the dark or the baryonic component.
We also show the first results from an unprecedented large-scale simulation of
500 Mpc/h and 2 times 512^3 gas and dark matter particles. This experiment will
make possible a detailed study of the large-scale distribution of clusters as a
function of their X-ray properties.
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No Label
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0711.2574
|
Michael Rissi mr
|
Upper limit for gamma-ray emission above 140 GeV from the dwarf
spheroidal galaxy Draco
|
astro-ph
|
The nearby dwarf spheroidal galaxy Draco with its high mass to light ratio is
one of the most auspicious targets for indirect dark matter searches.
Annihilation of hypothetical DM particles can result in high-energy gamma-rays,
e.g. from neutralino annihilation in the supersymmetric framework. With the
MAGIC telescope a search for a possible DM signal originating from Draco was
performed during 2007. The analysis of the data results in a flux upper limit
of 1.1x10^-11 photons cm^-2 sec^-1 for photon energies above 140 GeV, assuming
a point like source. Furthermore, a comparison with predictions from
supersymmetric models is given. While our results do not constrain the mSUGRA
phase parameter space, a very high flux enhancement can be ruled out.
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