HI 21 cm absorption in low z damped Lyman-α systems

Abstract

We report Giant Metrewave Radio Telescope (GMRT) 21cm observations of two confirmed and one candidate low redshift damped Lyman-α systems (DLAS). HI absorption was detected in the two confirmed systems, at z=0.2378 and z=0.5247, with spin temperatures (T_s) 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 3σ lower limit of 850 K on its spin temperature. Ground based K-band imaging observations (Turnshek et al. 2000) have identified the low T_s absorber with a spiral galaxy while the high T_s 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 (N_HI). Further, absorbers associated with spiral galaxies have systematically higher EW₂₁ at the same N_HI than DLAS with no such association, (2)DLAS at high redshift (z ≳ 2) have small velocity spreads (ΔV), while, at low redshift (z ≲ 2), both large and small ΔV are observed. These trends are in qualitative agreement with hierarchical models of galaxy formation.