The HI Companions of HII Galaxies and Low Surface Brightness Dwarf Galaxies

I study the VLA HI survey of HII galaxies by Taylor et al. (1995, ApJS, 99, 427; 1996, ApJS, 102, 189) and the VLA HI survey of low surface brightness (LSB) dwarf galaxies by Taylor et al. (1996, in press) to investigate the role of galaxy interactions in triggering the bursts of massive star formation seen in HII galaxies. Comparing the two surveys, I find that HII galaxies have companions more than twice as often as LSB dwarfs (p = 0.57 for HII galaxies, compared to p = 0.24 for LSB dwarfs). I examine the completeness of the companion samples detected by the two surveys. For the companions to HII galaxies, the sample is likely complete in the distribution of velocity separations from their parent galaxies, but is probably missing some companions at large projected linear separations because of the finite size of the VLA primary beam. For the companions of LSB dwarfs, the small number of detections means their distributions in velocity and linear separation are poorly determined, but the LSB dwarfs were observed with the same observational setup as the HII galaxies so they will have the same levels of completeness. Because the two samples were observed in exactly the same fashion, there will be no relative bias in the number of companions introduced in this way. In addition, the redshift distributions of the two samples are very similar, so there will not be a distance related relative bias. Thus I conclude that the difference in the number of HI rich companions is genuine, and signifies a difference in the local, small scale environments between the two types of galaxy. I search through published galaxy catalogs to determine number of neighbors each galaxy has outside the area of the VLA observations. At these large separations, the number of neighbors is the same, within the errors, for the two types of galaxy. The high rate of companion occurrence at low separations for HII galaxies relative to LSB dwarfs supports the hypothesis that the bursts of star formation are triggered by galaxy interactions.