Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: The Statistical and Physical Properties of the Low Redshift Lyman Alpha
  Forest Observed with HST/STIS   We examine the Ly-alpha absorber population at z<0.3 detected in spectra of
the QSOs PG0953+415 and H1821+643 taken with the Space Telescope Imaging
Spectrograph aboard the Hubble Space Telescope. We compare their statistical
properties to those in carefully-constructed mock quasar spectra drawn from a
cosmological hydrodynamic simulation of a Lambda-CDM universe. We find very
good agreement in the column density and b-parameter distributions, down to the
smallest observable absorbers with N_HI\approx 10^{12.3} cm^-2. The observed
absorber population is complete for N_HI\ga 10^{13} cm^-2, with a column
density distribution slope of \beta=2.04\pm 0.23 and a median b-parameter of 21
km/s above this limit. The intergalactic gas giving rise to these weak
absorbers is analogous to that at high redshift, located in diffuse large-scale
structures that are highly photoionized by the metagalactic UV flux, though a
greater number arise within shock-heated warm gas. The density, temperature,
and column density of these absorbers follow similar relationships to those at
high redshift, though with substantially larger scatter due to the shock-heated
gas. The b-parameters typically have a significant contribution from thermal
broadening, which facilitates a measurement of the low-z IGM temperature as
traced by Ly-alpha absorbers. From our simulation we estimate T_IGM\sim 5000 K,
with an upper limit of 10^4 K, at the mean density. The agreement in predicted
and observed amplitude of the column density distributions allows us to measure
the HI photoionization rate at z=0.17 to be 10^{-13.3\pm 0.7} s^{-1} (estimated
modeling uncertainty), close to predictions based on quasar properties.

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The categories of this paper are: solv-int
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The categories of this paper are: cmp-lg
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The categories of this paper are: acc-phys
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The categories of this paper are: hep-th
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The categories of this paper are: chem-ph
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The categories of this paper are: mtrl-th
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The categories of this paper are: atom-ph
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The categories of this paper are: q-bio
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The categories of this paper are: chao-dyn
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The categories of this paper are: eess
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The categories of this paper are: comp-gas
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The categories of this paper are: gr-qc
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The categories of this paper are: bayes-an
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The categories of this paper are: hep-lat
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The categories of this paper are: dg-ga
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The categories of this paper are: ao-sci