Very little is known about the spatial distribution of neutral gas in damped Lyman-# systems (DLAs), despite their
importance in understanding galaxy evolution. We propose a sensitive global VLBI experiment to simultaneously
map redshifted 21cm absorption in two low redshift DLAs towards B0738+313. Both absorbers have multiple 21cm
absorption components, in kinematically cold and warm gas phases, and the background QSO is known to be extended
over # 30 mas (# 50 pc and # 105 pc at the two DLA redshifts). The proposed observations will enable us to probe
the spatial distribution of the absorbing HI clouds, and to determine whether both the warm and cold phases cover
the entire radio source. In the lower redshift absorber, a comparison between the 21cm optical depths seen on VLBI
and WSRT baselines will also allow us to estimate or limit the HI mass of the host galaxy |