Diffuse neutral hydrogen in the H i Parkes All Sky Survey

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 H i beyond the ionisation edge of galaxy disks, column density sensitivities have to be achieved that probe the regime of Lyman limit systems. Typically H i observations are limited to a brightness sensitivity of N_HI ~ 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 H i Parkes All Sky Survey (HIPASS). HIPASS provides complete coverage of the region that has been observed for the Westerbork Virgo Filament H i 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.
Results: The newly processed data has an 1σ rms flux sensitivity of ~10 mJy beam^-1 over 26 km s^-1, corresponding to a column density sensitivity of ~3 × 10¹⁷ cm^-2. While the rms sensitivity is improved by only a modest 20%, the more substantial benefit is in the reduction of spectral artefacts near bright sources by more than an order of magnitude. In the reprocessed region we confirm all previously catalogued HIPASS sources and have identified 29 additional sources of which 14 are completely new H i detections. We derived spectra and moment maps for all detections together with total fluxes determined both by integrating the spectrum and by integrating the flux in the moment maps within the source radius. Extended emission or companions were sought in the nearby environment of each discrete detection. Ten extra-galactic filaments are marginally detected within the moment maps.
Conclusions: With the improved sensitivity after reprocessing and its large sky coverage, the HIPASS data is a valuable resource for detection of faint H i 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.

Appendix A is available in electronic form at http://www.aanda.org