High and low surface brightness galaxies in the local universe. V. Optical and HI properties

Using optical spectra of the nuclei of 250 galaxies chosen from the Automated Plate Measuring Facility (APM) survey of UK Schmidt plates, we derive synthetic B - V and V - R colors, estimates of reddening from Balmer decrements, metallicity via oxygen abundance, and star formation rate from the Ha luminosity. We also present mass-to-light ratios and gas mass fractions from 21 cm H I measurements of 238 APM survey galaxies, 101 of which also have optical spectra. This sample of galaxies spans a large range in surface brightness, from high surface brightness irregular galaxies down to the kind of low surface brightness (LSB) galaxies that are missing from most magnitude-limited catalogs. The generally blue global colors are best explained by a combination of low metallicity and active star formation, primarily because of the tremendous scatter in metallicity versus B - V. Optical spectra show the presence of G and K main-sequence stars in the nuclei of these galaxies, and the stellar mass-to-light ratios and gas mass fractions show the galaxies as a whole to be gas-rich. LSB galaxies are most likely to experience sporadic low levels of star formation over gigayear timescales.