HI 21cm Absorption at $z \sim 2.347$ towards PKS B0438-436

Details HI 21cm Absorption at $z \sim 2.347$ towards PKS B0438-436 HI 21cm Absorption at $z \sim 2.347$ towards PKS B0438-436

2006-05-14

arxiv.org/abs/astro-ph/0605346v1

Mon.Not.Roy.Astron.Soc.Lett. 370 (2006) L46-L50

Astronomy and Astrophysics

Astrophysics

5 pages, 5 figures; accepted for publication in MNRAS (Letters)

Scientific paper

10.1111/j.1745-3933.2006.00186.x

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.

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