Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Theory and Extragalactic Masers   Theoretical research on extragalactic masers (and hence this review) is
almost entirely directed toward 22 GHz water masers in circumnuclear disks with
the focus being the near ideal masing disk at the nucleus of the galaxy
NGC4258. The discussion here is organized around (1) the excitation and
conditions for the masers, (2) the spatial and spectral appearance of the maser
emission, and (3) the mass accretion rate and structure of the disk. In
addition, a summary is given of the basic physics (spectral linebeadths and
maser polarization) that underlies certain interpretations of these and other
astrophysical masers.

18
The categories of this paper are: econ
25
The categories of this paper are: patt-sol
2
The categories of this paper are: math
36
The categories of this paper are: cmp-lg
5
The categories of this paper are: gr-qc
21
The categories of this paper are: adap-org
37
The categories of this paper are: supr-con
13
The categories of this paper are: quant-ph
33
The categories of this paper are: plasm-ph
8
The categories of this paper are: hep-ex
14
The categories of this paper are: math-ph
4
The categories of this paper are: cond-mat
16
The categories of this paper are: stat
20
The categories of this paper are: acc-phys
30
The categories of this paper are: q-alg
35
The categories of this paper are: mtrl-th