The HI properties and environment of Lyman-α absorbers

We present results from two projects in which we have used the HI 21cm emission line as a tracer of gas-rich galaxy populations in the vicinity of Lyman-α absorbers. In the first case, we examine the HI environment of sbs 1543+593, the nearest damped Lyman-α absorber. We use a vla map of the region around this lsb galaxy which itself shows an extended HI disk to identify 3 gas-rich neighbours within 185 kpc. While it is not clear whether we should expect local damped Lyman-α systems to reside in such gas-rich regions, we would expect this kind of environment to be more prevalent at higher redshifts where less of the gas is in the dense inner regions of galaxies or has been consumed by star formation. This local galaxy is the only system in which we can study the gaseous environment in this kind of detail. In the second case, we examine the HI environment surrounding 16 Lyman-α forest absorbers along 4 QSO sight-lines. We do not detect any gas-rich galaxies at the absorber positions indicating that, at least down to our sensitivity limits, these absorption lines do not seem to be associated with galaxy halos. For half of the Lyman-α absorption systems there is a galaxy within 500 kpc, but for the other half there is not. In two cases there is no galaxy within 2 mpc of the Lyman-α absorption systems indicating that absorbers do, in some cases, reside in voids.