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

Abstract

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.

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

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