Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: HI 21cm Absorption at $z \sim 2.347$ towards PKS B0438-436   We report the detection of redshifted HI~21cm absorption in the $z \sim
2.347$ damped Lyman-$\alpha$ absorber (DLA) towards PKS B0438-436, with the
Green Bank Telescope. This is the second-highest redshift at which 21cm
absorption has been detected in a DLA. The absorption extends over $\sim 60$
km/s and has two distinct components, at $z = 2.347477 (12)$ and $z = 2.347869
(20)$. A similar velocity structure is seen in optical metal lines, although
the peak absorption here is offset by $\sim 11$ km/s from the peak in the 21cm
line. We obtain a high spin temperature $T_s \sim (886 \pm 248) \times
(f/0.58)$ K, using a covering factor estimated from 2.3 GHz VLBI data. However,
the current data cannot rule out a low spin temperature. The non-detection of
CO 3-2 absorption places the upper limit $N_{CO} < 3.8 \times 10^{15} \times
(T_x/10)$ cm$^{-2}$ on the CO column density.

25
The categories of this paper are: patt-sol
14
The categories of this paper are: math-ph
31
The categories of this paper are: ao-sci
1
The categories of this paper are: cs
20
The categories of this paper are: acc-phys
21
The categories of this paper are: adap-org
18
The categories of this paper are: econ
19
The categories of this paper are: eess
28
The categories of this paper are: alg-geom
12
The categories of this paper are: nucl-th
30
The categories of this paper are: q-alg
2
The categories of this paper are: math
34
The categories of this paper are: chem-ph
24
The categories of this paper are: dg-ga
26
The categories of this paper are: bayes-an
8
The categories of this paper are: hep-ex