The mass-to-light ratios of low surface brightness spiral galaxies: Clues from the tully-fisher relation

We have obtained 21 cm profiles and CCD surface photometry for a subset of field low surface brightness (LSB) spiral galaxies found by a large survey using the Automated Plate Measuring machine. We find that the LSB spirals generally follow the same Tully-Fisher relations defined by a sample of higher surface brightness (HSB) galaxies drawn from the Ursa Major cluster, albeit with a considerably greater scatter. This general trend implies that LSB galaxies of a given total luminosity have mass-to-light ratios (M/L) similar to those of HSB galaxies of comparable total luminosity, despite their differences in luminosity density (i.e., surface brightness). We also find evidence that galaxies with extremely large half-light radii (the "Malin 1 cousins") tend to be excessively luminous for their rotation speeds. We find that, at a given profile width, the luminosity of an LSB galaxy relative to the Tully-Fisher relation seems to be weakly anticorrelated with gas richness, indicating that some of the higher scatter may be associated with the evolutionary status of the LSB galaxies. Finally, we find that the LSB galaxies tend to have higher total atomic gas masses than the Ursa Major comparison galaxies, despite the generally comparable optical luminosities between the two sets.