... In this talk, I will discuss a study of the effect of ionization on the low end of HI mass function. The existence of a large population of low mass galaxies follows from CDM theory. ... HIDEEP found no low HI column density galaxies. One possible explanation is that the disks are "frozen". ... We assume that galaxies are exposed to an ionizing background radiation, from a uniform quasar distribution. ... We find a gap in the average column densities around N H which drops to 10 17 cm -2 . ...

... 2.3 Data acquisition Spectra generated by the Multibeam correlator contain the usual structure seen in spectra from other H i instruments: a bandpass spectrum which is the sum of the sky , ground and receiver temperatures multiplied by the product of the filters in the receiver chain; superimposed on this is a noise ... Note also that sources extended much beyond ,40 arcmin can be imaged poorly in HIPASS images because of the bandpass sidelobe effect described in Section 3.3. ...