A radio Search for high redshift HI absorption

Ground based optical observations have yielded considerable information on the statistics of damped-lyman alpha systems. In particular these systems are known to be the dominant repository of the observed neutral gas at high redshift. However, particularly at high redshift, there is the possibility that optical observations could be biased due to the exclusion of damped-lyman alpha systems that contain moderate to significant amounts of dust. Independent observational constraints on the neutral hydrogen content at high redshifts and the amount of dust in high redshift systems can be obtained from a radio search against the bright lobes of distant radio galaxies (which is less affected by the presence of dust in foreground damped-lyman alpha systems). We describe here a pilot radio survey along the line of sight to a small sample of high redshift radio galaxies, and also present some preliminary results. The survey uses a novel observing mode at the WSRT which enables one to make sensitive searches of a large redshift interval in a modest amount of telescope time.