Dissecting Galaxy Colors with GALEX, SDSS, and Spitzer

We combine data from the Sloan Digital Sky Survey (SDSS), the Galaxy Evolution Explorer (GALEX), and the Spitzer Space Telescope to create a sample of galaxies observed homogeneously from the UV to the far-IR. This sample, consisting of ~460 galaxies observed spectroscopically by the SDSS, provides us with a multiwavelength (0.15-24 μm) view of obscured and unobscured star formation in nearby (z < 0.3) galaxies with star formation rates (SFRs) ranging from 0.01 to 100 M☉ yr-1. We calculate a robust dust measure from the infrared-to-UV ratio (or infrared excess [IRX]) and explore the influence of star formation history (SFH) on the dust-UV color relation (i.e., the IRX-β relation). We find that the UV colors of galaxies are only weakly dependent on their SFH as measured by the 4000 Å break. However, we find that the contributions of dust and SFH are distinguishable when colors at widely separated wavelengths (e.g., 0.23-3.6 μm) are introduced. We show this explicitly by recasting the IRX-β relation as a more general IRX-SFH-color relation, which we examine in different projections. We also determine simple fits to this relation.