Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Adaptive Optics Imaging and Spectroscopy of Cygnus A: I. Evidence for a
  Minor Merger   We present Keck II adaptive optics near infrared imaging and spectroscopic
observations of the central regions of the powerful radio galaxy Cygnus A. The
0.05" resolution images clearly show an unresolved nucleus between two
spectacular ionization/scattering cones. We report the discovery of a
relatively bright (K'~19) secondary point source 0.4" or 400 pc in projection
southwest of the radio nucleus. The object is also visible in archival Hubble
Space Telescope optical images, although it is easily confused with the
underlying structure of the host. Although the near infrared colors of this
secondary point source are roughly consistent with those of an L-dwarf, its
spectrum and optical-to-infrared spectral energy distribution (SED) virtually
rule out the possibility that it may be any foreground projected object. We
conclude that the secondary point source is likely to be an extragalactic
object associated with Cygnus A. We consider several interpretations of the
nature of this object, including: a young star cluster peering through the dust
at the edge of one of the ionization cones; an older, large globular cluster; a
compact cloud of dust or electrons that is acting as a mirror of the hidden
active nucleus; and the dense core of a gas stripped satellite galaxy that is
merging with the giant elliptical host. The data presented here are most
consistent with the minor merger scenario. The spectra and SED of the object
suggest that it may be a densely packed conglomeration of older stars heavily
extincted by dust, and its high luminosity and compact nature are consistent
with those of a satellite that has been stripped to its tidal radius. Further
spectroscopic observations are nevertheless necessary to confirm this
possibility.

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The categories of this paper are: math
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The categories of this paper are: cmp-lg
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The categories of this paper are: alg-geom
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The categories of this paper are: hep-ex
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The categories of this paper are: chao-dyn
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The categories of this paper are: adap-org
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The categories of this paper are: hep-th
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The categories of this paper are: physics
30
The categories of this paper are: q-alg
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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: math-ph
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The categories of this paper are: quant-ph
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The categories of this paper are: ao-sci
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The categories of this paper are: bayes-an
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The categories of this paper are: nucl-ex