PROPERTIES OF AN H I-SELECTED GALAXY SAMPLE

We analyze the properties of a sample of galaxies identified in a 21 c m, H I-line survey of selected areas in the Perseus-Pisces supercluste r and its foreground void. Twelve fields were observed in the superclu ster, five of them (target fields) centered on optically bright galaxi es, and the other seven (blank fields) selected to contain no bright g alaxies within 45' of their centers. We detected nine previously uncat alogued, gas-rich galaxies, six of them in the target fields. We also detected H I from seven previously catalogued galaxies in these fields . Observations in the void covered the same volume as the 12 superclus ter fields at the same H I-mass sensitivity, but no objects were detec ted. Combining our H I data with optical broadband and Ha imaging, we conclude that the properties of H I-selected galaxies do not differ su bstantially from those of late-type galaxies found in optical surveys. In particular, the galaxies in our sample do not appear to be unusual ly faint for their H I mass, or for their circular velocity. We find t entative evidence for a connection between optical surface brightness and degree of isolation, in the sense that low surface brightness gala xies tend to be more isolated. The previously catalogued, optically br ight galaxies in our survey volume dominate the total H i mass density and cross section; the uncatalogued galaxies contribute only approxim ately 19% of the mass and approximately 12% of the cross section. Thus , existing estimates of the density and cross section of neutral hydro gen, most of which are based on optically selected galaxy samples, are probably accurate. Such estimates can be used to compare the nearby u niverse to the high-redshift universe probed by quasar