Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: 21 cm Absorption Studies with the Square Kilometer Array   HI 21~cm absorption spectroscopy provides an excellent probe of the neutral
gas content of absorbing galaxies, yielding information on their kinematics,
mass, physical size and ISM conditions. The high sensitivity, unrivaled
frequency coverage and RFI suppression techniques of the SKA will enable it to
use HI absorption to study the ISM of high column density intervening systems
along thousands of lines of sight out to high redshifts. Blind SKA 21~cm
surveys will yield large, unbiased absorber samples, tracing the evolution of
normal galaxies and active galactic nuclei from $z \gtrsim 6$ to the present
epoch. It will thus be possible to directly measure the physical size and mass
of typical galaxies as a function of redshift and, hence, to test hierarchical
models of structure formation.

31
The categories of this paper are: ao-sci
35
The categories of this paper are: mtrl-th
4
The categories of this paper are: cond-mat
13
The categories of this paper are: quant-ph
15
The categories of this paper are: nucl-ex
2
The categories of this paper are: math
33
The categories of this paper are: plasm-ph
24
The categories of this paper are: dg-ga
37
The categories of this paper are: supr-con
0
The categories of this paper are: hep-ph
14
The categories of this paper are: math-ph
9
The categories of this paper are: nlin
18
The categories of this paper are: econ
34
The categories of this paper are: chem-ph
11
The categories of this paper are: hep-lat
16
The categories of this paper are: stat