Star Formation in Hi–selected Galaxies. II. HiiRegion Properties

A sample of 69 galaxies with radial velocities less than 2500 km/s wasselected from the HI Parkes All Sky Survey (HIPASS) to deduce details aboutstar formation in nearby disk galaxies selected with no bias to optical surfacebrightness selection effects. Broad (B and R) and narrow band (H-alpha) imageswere obtained for all of these objects. More than half of the sample galaxiesare late-type, dwarf disks (mostly Sc and Sm galaxies). We have measured theproperties of the HII regions on H-alpha, continuum subtracted images, usingthe HIIphot package developed by Thilker et al. 2000. All but one of thegalaxies contained at least one detectable HII region. Examination of theproperties of the HII regions in each galaxy revealed that the brightestregions in higher surface brightness galaxies tend to be more luminous thanthose in lower surface brightness galaxies. A higher fraction (referred to asthe diffuse fraction) of the H-alpha emission from lower surface brightnessgalaxies comes from diffuse ionized gas (DIG). HII region luminosity functions(LFs) co-added according to surface brightness show that the shapes of the LFsfor the lowest surface brightness galaxies are different from those for typicalspiral galaxies. This discrepancy could be caused by the lowest surfacebrightness galaxies having somewhat episodic star formation or by them forminga relatively larger fraction of their stars outside of dense, massive molecularclouds. In general, the results imply that the conditions under which starformation occurs in lower surface brightness galaxies are different than inmore typical, higher surface brightness spiral galaxies.Comment: Accepted to the Astrophysical Journa