Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: A filamentary structure of massive star-forming galaxies associated with
  an X-ray absorbed QSO at z=1.8   The genesis of spheroids is central to our understanding of galaxy formation
-- they are relatively simple systems, containing about half the stellar mass
of the Universe. A major subset of spheroids, massive elliptical galaxies, are
preferentially found in clusters where they exhibit old, coeval stellar
populations suggesting that they formed synchronously at early epochs. Here we
report SCUBA submillimeter imaging of a region around a z=1.8 X-ray selected
QSO. The image reveals a remarkable 400 kiloparsec long chain of galaxies each
with an obscured star-formation rate sufficiently high to build a massive
spheroid in less than 1 Gyr. The large over-density of these galaxies relative
to expectations for a random field implies they probably reside in a structure
associated with the QSO. We suggest that this star formation is associated with
galaxy mergers or encounters within the filament, such as those predicted by
the popular hierarchical model of galaxy formation. Our observations suggest
that strong absorption in the X-ray spectra of QSOs at high-redshifts may
result from a veil of gas thrown up by a merger or merger-induced activity,
rather than an orientation-dependent obscuring torus. It is argued that these
systems are the precursors of elliptical galaxies found today in the core
regions of all rich galaxy clusters.

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The categories of this paper are: atom-ph
4
The categories of this paper are: cond-mat
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The categories of this paper are: q-alg
31
The categories of this paper are: ao-sci
36
The categories of this paper are: cmp-lg
7
The categories of this paper are: hep-th
35
The categories of this paper are: mtrl-th
12
The categories of this paper are: nucl-th
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The categories of this paper are: comp-gas
1
The categories of this paper are: cs
11
The categories of this paper are: hep-lat
5
The categories of this paper are: gr-qc
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The categories of this paper are: econ
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The categories of this paper are: funct-an
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The categories of this paper are: chao-dyn
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The categories of this paper are: plasm-ph