Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: The detectability of HI 21-cm absorption in damped Lyman-alpha systems   In this paper we investigate the possible reasons why HI 21-cm absorption in
damped Lyman-alpha systems (DLAs) has only been detected at low redshift... We
suggest that the lack of 21-cm absorption detections at high redshift arises
from the fact that these DLAs are at similar angular diameter distances to the
background quasars (i.e. the distance ratios are always close to unity): Above
z~1.6 the covering factor becomes largely independent of the DLA--QSO distance,
making the high redshift absorbers much less effective at covering the
background continuum emission. At low redshift, small distance ratios are
strongly favoured by the 21-cm detections, whereas large ratios are favoured by
the non-detections. This mix of distance ratios gives the observed mix of
detections and non-detections at z<1.6.In addition to the predominance of large
distance ratios and non-detections at high redshift, this strongly suggests
that the observed distribution of 21-cm absorption in DLAs is dominated by
geometric effects.

23
The categories of this paper are: solv-int
14
The categories of this paper are: math-ph
32
The categories of this paper are: atom-ph
25
The categories of this paper are: patt-sol
9
The categories of this paper are: nlin
36
The categories of this paper are: cmp-lg
26
The categories of this paper are: bayes-an
27
The categories of this paper are: comp-gas
7
The categories of this paper are: hep-th
18
The categories of this paper are: econ
10
The categories of this paper are: q-bio
29
The categories of this paper are: funct-an
30
The categories of this paper are: q-alg
3
The categories of this paper are: physics
22
The categories of this paper are: chao-dyn
8
The categories of this paper are: hep-ex