HI 21cm absorption in low z damped lyman-alpha systems

Abstract

We report Giant Metrewave Radio Telescope (GMRT) 21cm observations of two confirmed and one candidate low redshift damped Lyman-alpha systems (DLAS). HI absorption was detected in the two confirmed systems, at z=0.2378 and z=0.5247, with spin temperatures (Ts) of 2050 +- 200 K and 330 +- 70 K respectively. The non-detection of absorption in the third (candidate) system places an upper limit on its optical depth, which in turn implies a 3sigma lower limit of 850 K on its spin temperature. Ground based K-band imaging observations (Turnshek et al. 2000) have identified the low Ts absorber with a spiral galaxy while the high Ts system appears to be an LSB galaxy. This continues the trend noted earlier by Chengalur and Kanekar (2000) of low spin temperatures for DLAS associated with spiral galaxies and high spin temperatures for those with no such association. Combining these new results with 21cm observations from the published literature, we find that (1)The 21 cm equivalent width (EW21) of DLAS increases with increasing column density (NHI). Further, absorbers associated with spiral galaxies have systematically higher EW21 at the same NHI than DLAS with no such association, (2)DLAS at high redshift (z > 2) have small velocity spreads (DeltaV), while, at low redshift (z < 2), both large and small DeltaV are observed. These trends are in qualitative agreement with hierarchical models of galaxy formation.