The Properties of an HI-Selected Galaxy Sample
  We analyze the properties of a sample of galaxies identified in a 21-cm, HI-line survey of selected areas in the Perseus-Pisces supercluster and its foreground void. Twelve fields were observed in the supercluster, five of them (target fields) centered on optically bright galaxies, and the other seven (blank fields) selected to contain no bright galaxies. We detected nine previously uncataloged, gas-rich galaxies, six of them in the target fields. We also detected HI from seven previously cataloged galaxies in these fields. Observations in the void covered the same volume as the twelve supercluster fields at the same HI-mass sensitivity, but no objects were detected. Combining our HI data with optical broad-band and H-alpha imaging, we conclude that the properties of HI-selected galaxies do not differ substantially from those of late-type galaxies found in optical surveys. In particular, the galaxies in our sample do not appear to be unusually faint for their HI mass, or for their circular velocity. The previously cataloged, optically bright galaxies in our survey volume dominate the total HI mass density and cross-section; the uncataloged galaxies contribute only 19% of the mass and 12% of the cross-section. Thus, existing estimates of the density and cross-section of neutral hydrogen, most of which are based on optically-selected galaxy samples, are probably accurate. 
A. Szomoru, P. Guhathakurta, J. van Gorkom, J. Knapen, D. Weinberg &   A. Fruchter