Stellar and gas properties of high H I mass-to-light ratio galaxies in the local universe

We present a multiwavelength study (BVRI-band photometry and H I line interferometry) of nine late-type galaxies selected from the HIPASS Bright Galaxy Catalog on the basis of apparently high H I mass-to-light ratios (3 M _⊙/L _⊙,B < M _{H I}/L _B < 27 M _⊙/L _⊙,B). We find that most of the original estimates for M _{H I}/L _B based on available photographic magnitudes in the literature were too high and conclude that genuine high H I mass-to-light ratio (>5 M _⊙/L _⊙,B) galaxies are rare in the local universe. Extreme high-M _HI\L _B galaxies such as ESO 215-G?009 appear to have formed only the minimum number of stars necessary to maintain the stability of their H I disks and could possibly be used to constrain galaxy formation models. They may also have been forming stars at a low, constant rate over their lifetimes. The best examples all have highly extended H I disks, are spatially isolated, and have normal baryonic content for their total masses but are deficient in stars. This suggests that high-M _{H I}/L _B galaxies are not lacking the baryons to create stars but are underluminous, as they lack either the internal or external stimulation for more extensive star formation.