STRUCTURAL CHARACTERISTICS AND STELLAR COMPOSITION OF LOW SURFACE BRIGHTNESS DISK GALAXIES

We present UBVI surface photometry of a sample of low surface brightne ss (LSB) disk galaxies. LSB disk galaxies are fairly well described as exponential disks with no preferred value for either scale length, ce ntral surface brightness, or rotational velocity. Indeed, the distribu tion of scale lengths is indistinguishable from that of high surface b rightness spirals, indicating that dynamically similar galaxies (e.g., those with comparable upsilon(2)R) exist over a large range in surfac e density. These LSB galaxies are strikingly blue. The complete lack o f correlation between central surface brightness and color rules out a ny fading scenario. Similarly, the oxygen abundances inferred from H I I region spectra are uncorrelated with color so the low metallicities are not the primary cause of the blue colors. While these are difficul t to interpret in the absence of significant star formation, the most plausible scenario is a stellar population with a young mean age stemm ing from late formation and subsequent slow evolution. These propertie s suggest that LSB disks formed from low initial overdensities with co rrespondingly late collapse times.