The properties of ultraviolet-luminous galaxies at the current EPOCH

We have used the first matched set of Galaxy Evolution Explorer (GALEX) and Sloan Digital Sky Survey (SDSS) data to investigate the properties of a sample of 74 nearby (z < 0.3) galaxies with far-ultraviolet luminosities greater than 2 × 10¹⁰ L_⊙, chosen to overlap the luminosity range of typical high-z Lyman break galaxies (LBGs). GALEX deep surveys have shown that ultraviolet-luminous galaxies (UVLGs) similar to these are the fastest evolving component of the UV galaxy population. Model fits to the combined GALEX and SDSS photometry yield typical FUV extinctions in UVLGs of 0.5-2 mag (similar to LBGs and less luminous GALEX-selected galaxies). The implied star formation rates are SFR ∼3-30 M_⊙ yr ^-1. This overlaps the range of SFRs for LBGs. We find a strong inverse correlation between galaxy mass and FUV surface brightness, and on this basis we divide the sample into "large" and "compact" UVLGs. The large UVLGs are relatively massive (M.̃ 10¹¹ M _⊙) late-type disk galaxies forming stars at a rate similar to their past average (M_*/SFR ̃ tHubble), They are matel rich (approximately solar), have intermediate optical-UV colors (FUV - r ∼ 2-3), and about a third host a type 2 (obscured) active galactic nucleus. In contrast, the compact UVLGs have half-light radii of a few kpc or less (similar to LBGs ). They are relatively low-mass galaxies (M_* ̃ 10¹⁰ M_⊙) with typical velocity dispersions of 60-150 km s^-1. They span a range in metallicity from ∼0.3 to 1 times solar, have blue optical-UV colors (FUV - r ∼ 0.5-2), and are forming stars at a rate sufficient to build the present galaxy in ∼l-2 Gyr. In all these respects they appear similar to the LBG population. These "living fossils" may therefore provide an opportunity for detailed investigation of the physical processes occurring in typical star-forming galaxies in the early universe.