H I 21 cm absorption at z~ 2.347 towards PKS B0438-436

Abstract

We report the detection of redshifted H i 21-cm absorption in the z^~2.347 damped Lyman 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⁻¹ and has two distinct components, at z= 2.347 477(12) and z= 2.347 869(20). A similar velocity structure is seen in optical metal lines, although the peak absorption here is offset by ~11 km s⁻¹ from the peak in the 21-cm line. We obtain a high spin temperature T_s~ (886 ±248) × (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 < 3.8 × 10¹⁵ (T_x/10) cm⁻² on the CO column density.