Star Formation in Hi–Selected Galaxies. I. Sample Characteristics

A sample of 69 galaxies with radial velocities less than 2500 km/s wasselected from the HI Parkes All Sky Survey (HIPASS) and imaged in broad band Band R and narrow band H-alpha to deduce details about star formation in nearbydisk galaxies while avoiding surface brightness selection effects. The sampleis dominated by late-type, dwarf disks (mostly Sc and Sm galaxies) withexponential disk scale lengths of about 1 to 5 kpc. The HIPASS galaxies onaverage have lower star formation rates (SFRs) and are bluer and lower surfacebrightness than an optically selected sample. HII regions were detected in allbut one of the galaxies. Many galaxies had as few as two to five HII regions.The galaxies' H-alpha equivalent widths, colors, and SFRs per unit HI mass arebest explained by young mean ages (about 3 to 5 Gyr according to Schmidt Lawmodels) with star formation histories where the SFRs were higher in the past.Comparison of the surface brightness coverage of the HIPASS galaxies with thatof an optically selected sample shows that such a sample may miss about 10% ofthe local galaxy number density and could possibly miss as much as 3 to 4% ofthe SFR density. The amount lower surface brightness galaxies contribute to thetotal luminosity density may be insignificant, but this conclusion is somewhatdependent on how the fluxes of these objects are determined.Comment: Accepted to the Astrophysical Journa