Gas-rich galaxies and the H i mass function

Abstract

We have developed an automated cross-correlation technique to detect 21cm 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, Davies and Smith. The galaxies were originally selected to have properties similar to Fornax cluster dE galaxies. As dE galaxies are generally gas poor it is not surprising that there were only 26 secure detections. All of the detected galaxies have very high values of (MH/LB)⊙. Thus the HI 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. These objects appear to have relative dark matter content similar to that of optically-selected galaxies. The Hi-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 HI mass function by looking at the distribution of velocities of HI detections in random spectra on the sky. The inferred HI mass function is steep, though confirmation of this result awaits a detailed study of the noise characteristics of the HI survey.