Star Formation in H I-Selected Galaxies. I. Sample Characteristics

A sample of 69 galaxies with radial velocities of less than 2500 km s^-1 was selected from the H I Parkes All-Sky Survey (HIPASS) and imaged in broadband B and R and narrowband Hα, to deduce details about star formation in nearby disk galaxies while avoiding surface brightness selection effects. The sample is dominated by late-type, dwarf disks (mostly Sc and Sm galaxies) with exponential disk scale lengths of ~1-5 kpc. The HIPASS galaxies, on average, have lower star formation rates (SFRs), are bluer, and have lower surface brightness than an optically selected sample. H II regions were detected in all but one of the galaxies. Many galaxies had as few as two to five H II regions. The galaxies' Hα equivalent widths, colors, and SFRs per unit of H I mass are best explained by young mean ages (~3-5 Gyr, according to Schmidt-law models) with star formation histories in which the SFRs were higher in the past. Comparison of the surface brightness coverage of the HIPASS galaxies with that of an optically selected sample shows that such a sample may miss ~10% of the local galaxy number density and could possibly miss as much as 3%-4% of the SFR density. The amount lower surface brightness galaxies contribute to the total luminosity density may be insignificant, but this conclusion is somewhat dependent on how the fluxes of these objects are determined.