2006MNRAS.370L..46K

We report the detection of redshifted HI 21-cm absorption in the z ∼ 2.347 damped Lyman α absorber (DLA) towards PKS B0438-436 with the Green Bank Telescope. This is the second-highest redshift at which 21-cm absorption has been detected in a DLA. The absorption extends over ∼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 ∼11 km/s from the peak in the 21-cm line. We obtain a high spin temperature T_s ∼ (886±248)x(f/0.58) K, using a covering factor estimated from 2.3-GHz very long baseline interferometry 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< 3.8x10¹⁵ (T_x/10)/cm2 on the CO column density.