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.