Giant Metrewave Radio Telescope Detection Of Two New H I 21 Cm Absorbers At Z ≈ 2

Abstract

I report the detection of H i 21 cm absorption in two high column density damped Lyα absorbers (DLAs) at z ≈ 2 using new wide-band 250–500 MHz receivers on board the Giant Metrewave Radio Telescope. The integrated H i 21 cm optical depths are 0.85 ± 0.16 km s−1 (TXS1755+578) and 2.95 ± 0.15 km s−1 (TXS1850+402). For the z = 1.9698 DLA toward TXS1755+578, the difference in H i 21 cm and C i profiles and the weakness of the radio core suggest that the H i 21cm absorption arises toward radio components in the jet, and that the optical and radio sightlines are not the same. This precludes an estimate of the DLA spin temperature. For the z = 1.9888 DLA toward TXS1850+402, the absorber covering factor is likely to be close to unity, as the background source is extremely compact, with the entire 5 GHz emission arising from a region of ≤ 1.4 mas in size. This yields a DLA spin temperature of Ts = (372 ± 18) × (f/1.0) K, lower than typical Ts values in high-z DLAs. This low spin temperature and the relatively high metallicity of the z = 1.9888 DLA ([Zn/H] =(− 0.68 ± 0.04)) are consistent with the anti-correlation between metallicity and spin temperature that has been found earlier in damped Lyα systems.