Using HI to probe large scale structures at z similar to 3

The redshifted 1420 MHz emission from the Hi in unresolved damped Lyman-alp ha clouds at high z will appear as la background radiation in low frequency radio observations. This holds the possibility of a new tool for studying the universe at high-z, using the mean brightness temperature to probe the HI content and its fluctuations to probe the power spectrum. Existing estim ates of the HI density at z similar to 3 imply a mean brightness temperatur e of 1 mK at 320 MHz. The cross-correlation between the temperature fluctua tions across different frequencies and sight lines is predicted to vary fro m 10(-7) K-2 to 10(-8) K-2 over intervals corresponding to spatial scales,f rom 10 Mpc to 40 Mpc for some of the currently favoured cosmological models . Comparing this with the expected sensitivity of the GMRT, we find that th is can be detected with similar to 10 hrs of integration, provided we can d istinguish it from the galactic and extragalactic foregrounds which will sw amp this signal. We discuss a strategy based dd the very distinct spectral properties of the foregrounds as against the HI emission, possibly allowing the removal of the foregrounds from the observed maps.