Giant Metrewave Radio Telescope detection of two new HI 21cm absorbers at $z \approx 2$

Abstract

I report the detection of HI 21cm absorption in two high column density damped Lyman-$\alpha$ absorbers (DLAs) at $z \approx 2$ using new wide-band $250-500$ MHz receivers onboard the Giant Metrewave Radio Telescope. The integrated HI 21cm optical depths are $0.85 \pm 0.16$ km/s (TXS1755+578) and $2.95 \pm 0.15$ km/s (TXS1850+402). For the $z=1.9698$ DLA towards TXS1755+578, the difference in HI 21cm and CI profiles and the weakness of the radio core suggest that the HI 21cm absorption arises towards 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 towards TXS1850+402, the absorber covering factor is likely to be close to unity, as the background source is extremely compact, with all the 5 GHz emission arising from a region of size $\leq 1.4$ mas. This yields a DLA spin temperature of ${\rm T_s} = (372 \pm 18) \times (f/1.0)$ K, lower than typical ${\rm T_s}$ values in high-$z$ DLAs. This low spin temperature and the relatively high metallicity of the $z = 1.9888$ DLA ([Zn/H] $= (-0.68 \pm 0.04)$) are consistent with the anti-correlation between metallicity and spin temperature that has been earlier found in damped Lyman-$\alpha$ systems.