The diverse properties of the most ultraviolet-luminous galaxies discovered by GALEX

We report on the properties of a sample of ultraviolet-luminous galaxies (UVLGs) selected by matching the Galaxy Evolution Explorer (GALEX) All-Sky Imaging and Medium Imaging Surveys with the Sloan Digital Sky Survey third data release. The overlap between these two surveys is roughly 450 deg(2). Of 25,362 galaxies ( with SDSS spectroscopy) in the range 0.0 < z < 0.3 detected by GALEX, there are 215 galaxies with L > 2 x 10(10) L-circle dot at 1530 angstrom (observed wavelength). The properties of this population are well correlated with ultraviolet surface brightness. We find that the galaxies with low UV surface brightness are primarily large spiral systems with a mixture of old and young stellar populations, while the high surface brightness galaxies consist primarily of compact starburst systems, with an approximate boundary at a surface brightness of I-1530 = 10(8) L-circle dot kpc(-2). The large galaxies appear to be the high-luminosity tail of the galaxy star formation function and owe their large luminosity to their large surface area. In terms of the behavior of surface brightness with luminosity, size with luminosity, the mass-metallicity relation, and other parameters, the compact UVLGs clearly depart from the trends established by the full sample of galaxies. The subset of compact UVLGs with the highest surface brightness (I-1530 > 10(9) L-circle dot kpc(-2); supercompact UVLGs) have characteristics that are remarkably similar to Lyman break galaxies at higher redshift. They are much more luminous ( and thus have much higher star formation rates) than typical local ultraviolet-bright starburst galaxies and blue compact dwarf galaxies. They have metallicities that are systematically lower than normal galaxies of the same stellar mass, indicating that they are less chemically evolved. In all these respects, they are the best local analogs for Lyman break galaxies.