Diffuse neutral hydrogen in the Hi Parkes All Sky Survey

Abstract

Context. Observations of neutral hydrogen can provide a wealth of information about the distribution and kinematics of galaxies. 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. Aims. 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 NHI ∼ 1019 cm-2 but this has to be improved by at least an order of magnitude. Methods. In this paper, reprocessed data is presented that was originally observed for the Hi Parkes All Sky Survey (HIPASS).HIPASS provides complete coverage of the region that has been observed for the Westerbork Virgo Filament Hi Survey (WVFS), presented in accompanying papers, and thus is an excellent product for data comparison. The region of interest extends from 8 to 17 hours in right ascension and from -1 to 10 degrees in declination. Although the original HIPASS product already has good flux sensitivity, the sensitivity and noise characteristics can be significantly improved with a different processing method. Conclusions. With the improved sensitivity after reprocessing and its large sky coverage, the HIPASS data is a valuable resource for detection of faint Hi emission. This faint emission can correspond to extended halos, dwarf galaxies, tidal remnant and potentially diffuse filaments that represent the trace neutral fraction of the Cosmic Web.