Giant metrewave radio telescope observations of low-z damped Lyα absorbers

Abstract

We present Giant Metrewave Radio Telescope (GMRT) observations of redshifted H i 21-cm absorption in two low-redshift (z=0.2212, z=0.0912) damped Ly α systems seen towards the GHz peaked source OI 363 (z_em = 0.630). The object at z=0.0912 is the lowest redshift damped Lyα system known to date. Ground-based imaging by Rao & Turnshek shows that at neither redshift is there a large spiral galaxy at low impact parameter to the line of sight to OI 363, in contradiction with the suggestion that damped Lyα systems are large protodiscs. Since OI 363 is a highly compact, core-dominated source, the covering factor of the H i gas is likely to be unity. Nonetheless, the spin temperatures derived from the 21-cm optical depth (and using the N_H,i measured from Hubble Space Telescope spectra) are high, namely 1120 ± 200 K and 825 ± 110 K for the high- and low-redshift systems respectively. These values are considerably higher than typical values (100{−}200 K) measured in our Galaxy and Andromeda and are, in fact, similar to those obtained in high-redshift damped Ly systems. Our observations hence suggest that evolutionary effects may not be crucial in understanding the difference in derived spin temperature values between local spiral discs and high-redshift damped Lyα systems.