Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Kinematic and Chemical constraints on the formation of M31's inner and
  outer halo   The halo of M31 shows a wealth of substructures that are consistent with
satellite accretion. Here we report on kinematic and abundance results from
Keck/DEIMOS spectroscopy in the calcium triplet region of over 3500 red giant
star candidates along the minor axis and in off-axis spheroid fields of M31.
Our data reach out to large radial distances of 160 kpc. The derived velocity
distributions show a kinematically cold substructure at 17 kpc that has been
reported before. We devise an improved method to measure accurate metallicities
from the calcium triplet in low signal-to-noise spectra using a coaddition of
the individual lines. The resulting distribution leads us to note an even
stronger gradient in the abundance distribution along M31's minor axis than
previously detected. The mean metallicity in the outer halo reaches below -2
dex, with individual values as low as -2.6 dex. In the inner spheroid, at 17-19
kpc, we find a sharp decline of ~0.5 dex in metallicity, which roughly
coincides with the edge of an extended disk, previously detected from star
count maps. A large fraction of red giants in the most distant fields are
likely members of M33's overlapping halo. A comparison of our velocities with
those predicted by new N-body simulations argues that the event responsible for
the giant Stream is most likely not responsible for the full population of the
inner halo; we show further that the abundance distribution of the Stream is
different from that of the inner halo, from which it becomes evident that the
merger event that formed the outer halo cannot have contributed any significant
material to the inner spheroid. All this evidence of severe structure changes
in the halo suggests a high degree of infall and stochastic abundance accretion
governing the build-up of M31's halo. (Abridged)

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The categories of this paper are: econ
14
The categories of this paper are: math-ph
25
The categories of this paper are: patt-sol
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The categories of this paper are: mtrl-th
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The categories of this paper are: acc-phys
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The categories of this paper are: hep-lat
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The categories of this paper are: cmp-lg
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The categories of this paper are: q-bio
2
The categories of this paper are: math
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The categories of this paper are: q-alg
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The categories of this paper are: hep-ex
4
The categories of this paper are: cond-mat
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The categories of this paper are: eess
0
The categories of this paper are: hep-ph
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The categories of this paper are: quant-ph
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The categories of this paper are: stat