Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: The 21cm "Outer Arm" and the Outer-Galaxy High-Velocity Clouds:
  Connected by Kinematics, Metallicity, and Distance   Using high-resolution ultraviolet spectra obtained with the HST/Space
Telescope Imaging Spectrograph (STIS) and the Far Ultraviolet Spectroscopic
Explorer, we study the metallicity, kinematics, and distance of the gaseous
"Outer Arm" (OA) and the high-velocity clouds (HVCs) in the outer Galaxy. We
detect the OA in a variety of absorption lines toward two QSOs, H1821+643 and
HS0624+6907. We search for OA absorption toward eight Galactic stars and detect
it in one case, which constrains the OA Galactocentric radius to 9<R_{G}<18
kpc. We also detect HVC Complex G, which is projected near the OA at a similar
velocity, in absorption toward two stars; Complex G is therefore in the same
region at R_{G} = 8 - 10 kpc. HVC Complex C is known to be at a similar
Galactocentric radius. Toward H1821+643, the low-ionization absorption lines
are composed of multiple narrow components, indicating the presence of several
cold clouds and rapid cooling and fragmentation. Some of the highly ionized gas
is also surprisingly cool. Accounting for ionization corrections, we find that
the OA metallicity is Z=0.2-0.5 Z_{solar}, but nitrogen is underabundant and
some species are possibly mildly depleted by dust. The similarity of the OA
metallicity, Galactocentric location, and kinematics to those of the adjacent
outer-Galaxy HVCs, including high velocities that are not consistent with
Galactic rotation, suggests that the OA and outer-Galaxy HVCs could have a
common origin.

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The categories of this paper are: physics
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The categories of this paper are: patt-sol
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The categories of this paper are: nucl-ex
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The categories of this paper are: stat
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The categories of this paper are: hep-ph
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The categories of this paper are: plasm-ph
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The categories of this paper are: math
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The categories of this paper are: supr-con
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The categories of this paper are: quant-ph
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The categories of this paper are: funct-an
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The categories of this paper are: math-ph
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The categories of this paper are: acc-phys
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The categories of this paper are: nucl-th
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The categories of this paper are: hep-ex
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The categories of this paper are: eess
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The categories of this paper are: q-alg