GMRT Observations of Low z Damped Lyα Absorbers

We present Giant Metrewave Radio Telescope (GMRT) observations of redshifted HI 21cm absorption in two low redshift (z = 0.2212, z = 0.0912) damped Lyα systems seen towards the Gigahertz peaked source OI 363 (zem = 0.630). The object at z = 0.0912 is the lowest redshift damped Lyα system known to date. Ground based imaging (Rao & Turnshek 1998) 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 proto-disks. Since OI 363 is a highly compact, core dominated source, the covering factor of the HI gas is likely to be unity. Nonetheless, the spin temperatures derived from the 21cm optical depth (and using the NHI measured from HST spectra (Rao & Turnshek 1998)) are high, viz. 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 Lyα systems. Our observations hence suggest that evolutionary effects may not be crucial in understanding the difference in derived spin temperature values between local spiral disks and high redshift damped Lyα systems.