Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Subparsec-scale HI in the nucleus of NGC 4151   We present sensitive, high-resolution 21-cm VLBA+VLA observations of the
radio jet and nuclear HI absorption in NGC 4151. The 25-mas (1.6-pc) resolution
continuum image reveals a highly collimated radio jet, underlying the discrete
components seen previously with MERLIN and the VLA. Spatially and kinematically
complex HI absorption is detected against the whole 3-pc extent of the
continuum component predicted by Ulvestad et al. to contain the AGN. Instead,
we suggest the component against which the absorption is detected is part of
the eastern counterjet, ruling it out as the location for the AGN.

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The categories of this paper are: physics
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The categories of this paper are: atom-ph
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The categories of this paper are: plasm-ph
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The categories of this paper are: adap-org
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The categories of this paper are: cmp-lg
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The categories of this paper are: comp-gas
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The categories of this paper are: chem-ph
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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: gr-qc
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The categories of this paper are: stat
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The categories of this paper are: nucl-ex
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The categories of this paper are: ao-sci
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The categories of this paper are: nucl-th
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The categories of this paper are: nlin
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The categories of this paper are: math-ph