Diffuse neutral hydrogen in HIPASS

To learn more about large scale structures and accretion processes, the extended environment of galaxies must also be observed. Numerical simulations predict a cosmic web of extended structures and gaseous filaments.

To detect HI beyond the ionisation edge of galaxy disks, column density sensitivities have to be achieved that probe the regime of Lyman limit systems. Typically HI observations are limited to a brightness sensitivity of N_HI_~10^19^cm^-2^ but this has to be improved by at least an order of magnitude.