Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Probing the nature of the ISM in Active Galactic Nuclei through HI
  absorption   The physical and kinematical conditions of the gas surrounding an active
galactic nucleus (AGN) offer key diagnostics for understanding the processes
occurring in the inner few kpc around the nucleus. Neutral hydrogen can give
important insights on these regions. Apart from probing the presence of gas in
relatively settled conditions (i.e. circumnuclear disks/tori) it can also trace
the presence of extreme outflows. Some examples of these phenomena are briefly
presented. For the study of the neutral hydrogen around AGN the high resolution
offered by the VLBI is crucial in order to locate the regions where the
absorption occurs and to study in detail the kinematics of the gas. Recent VLBI
results are discussed here.

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The categories of this paper are: bayes-an
8
The categories of this paper are: hep-ex
1
The categories of this paper are: cs
10
The categories of this paper are: q-bio
19
The categories of this paper are: eess
14
The categories of this paper are: math-ph
27
The categories of this paper are: comp-gas
32
The categories of this paper are: atom-ph
22
The categories of this paper are: chao-dyn
9
The categories of this paper are: nlin
15
The categories of this paper are: nucl-ex
25
The categories of this paper are: patt-sol
3
The categories of this paper are: physics
35
The categories of this paper are: mtrl-th
24
The categories of this paper are: dg-ga
34
The categories of this paper are: chem-ph