Giant Metrewave Radio Telescope observations of low-z damped Ly 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 (zem = 0.630). The object at z=0.0912 is the lowest redshift damped Ly α 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 Ly α 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 α 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 Ly α systems.