HIDEEP - An extragalactic blind survey for very low column-density neutral hydrogen

We have carried out an extremely long integration time (9000 s beam ^-1) 21-cm blind survey of 60 deg ² in Centaurus using the Parkes multibeam system. We find that the noise continues to fall as √t _obs throughout, enabling us to reach an HI column-density limit of 4.2 × 10 ^-18 cm ^-2 for galaxies with a velocity width of 200 km s ^-1 in the central 32 deg ² region, making this the deepest survey to date in terms of column density sensitivity. The HI data are complemented by very deep optical observations from digital stacking of multi-exposure UK Schmidt Telescope R-band films, which reach an isophotal level of 26.5 R mag arcsec ^-2 (≃27.5 B mag arcsec ^-2). 173 HI sources have been found, 96 of which have been uniquely identified with optical counterparts in the overlap area. There is not a single source without an optical counterpart. Although we have not measured the column densities directly, we have inferred them from the optical sizes of their counterparts. All appear to have a column density of N _HI = 10 ^20.65±0.38 This is at least an order of magnitude above our sensitivity limit, with a scatter only marginally larger than the errors on N _HThis needs explaining. If confirmed it means that HI surveys will only find low surface brightness (LSB) galaxies with high M _HI/L _B. Gas-rich LSB galaxies with lower Hi mass to light ratios do not exist. The paucity of low column-density galaxies also implies that no significant population will be missed by the all-sky HI surveys being carried out at Parkes and Jodrell Bank.