Gas rich galaxies and the HI mass function

We have developed an automated cross-correlation technique to detect 21-cm emission in sample spectra obtained from the H I Parkes All Sky Survey. The initial sample selection was the nearest spectra to 2435 low surface brightness galaxies in the catalogue of Morshidi-Esslinger et al. (1999). All 26 of the detected galaxies have very high values of (M-HI/L-B)circle dot. Thus the H I selection of faint optical sources leads to the detection of predominately gas-rich galaxies. The gas-rich galaxies tend to reside on the outskirts of the large-scale structure delineated by optically selected galaxies, but they do appear to be associated with it. The H I column densities are lower than the 'critical density' necessary for sustainable star formation and they appear, relatively, rather isolated from companion galaxies. These two factors may explain their high relative gas content. We have considered the H I mass function by looking at the distribution of velocities of H I detections in random spectra on the sky. The inferred H I mass function is steep, though confirmation of this result awaits a detailed study of the noise characteristics of the H I survey.