The Physical Properties of Low Surface Brightness Galaxies

Low Surface Brightness (LSB) galaxies are those with central disk surface brightnesses many standard deviations fainter than the canonical value. Severe selection effects stemming from the contrast relative to the arbitrary level of the sky background act to limit knowledge of the LSB component of the galaxy population. This component could be numerically dominant, yet very little is known about the physical properties of such galaxies. This situation is addressed by making a detailed examination of a large sample of LSB galaxies. Structurally, LSB galaxies are similar to high surface brightness (HSB) spiral galaxies. They are comparable in size, and span an enormous range in both surface brightness and luminosity. There is no correlation between size and surface brightness, and galaxies occupy the entire plane defined by these parameters. Physically, LSB galaxies are observed to typically be metal poor, very blue, and possess large gas mass fractions and small mass to light ratios. These indicate a relatively unevolved state, with less of the original gas converted into stars than in HSB galaxies. The blue colors are extreme and are difficult to understand, but probably indicate stellar populations with young mean ages and underpopulated giant branches. There are some cases of galaxies which may be undergoing their first episode of star formation. The elemental abundance ratios indicate either very peculiar star formation histories and IMFs, or some external source of enrichment. The latter provides a better empirical fit, but is not readily explicable without altering standard primordial nucleosynthesis. The low degree of evolution of LSB galaxies is caused by the low surface densities of their atomic gas disks. These remain predominantly below the threshold for star formation, so that this occurs only sporadically, and evolution proceeds gradually. The low densities of LSB galaxies are a natural consequence of their formation from modest initial density fluctuations. From this, it follows that LSB galaxies should be common at all masses, though they may be subject to destruction in high density environments. Thus, observational selection effects may indeed have severely limited knowledge of the majority of the galaxy population.