The Ultraviolet, Optical, and Infrared Properties of Sloan Digital Sky Survey Sources Detected byGALEX

We discuss the UV, optical, and IR properties of the SDSS sources detected byGALEX as part of its All-sky Imaging Survey Early Release Observations.Virtually all of the GALEX sources in the overlap region are detected by SDSS.GALEX sources represent ~2.5% of all SDSS sources within these fields and abouthalf are optically unresolved. Most unresolved GALEX/SDSS sources are brightblue turn-off thick disk stars and are typically detected only in the GALEXnear-UV band. The remaining unresolved sources include low-redshift quasars,white dwarfs, and white dwarf/M dwarf pairs, and these dominate the opticallyunresolved sources detected in both GALEX bands. Almost all the resolved SDSS sources detected by GALEX are fainter than theSDSS 'main' spectroscopic limit. These sources have colors consistent withthose of blue (spiral) galaxies (u-r<2.2), and most are detected in both GALEXbands. Measurements of their UV colors allow much more accurate and robustestimates of star-formation history than are possible using only SDSS data.Indeed, galaxies with the most recent (<20 Myr) star formation can be robustlyselected from the GALEX data by requiring that they be brighter in the far-UVthan in the near-UV band. However, older starburst galaxies have UV colorssimilar to AGN, and thus cannot be selected unambiguously on the basis of GALEXfluxes alone. With the aid of 2MASS data, we construct and discuss median 10 bandUV-optical-IR spectral energy distributions for turn-off stars, hot whitedwarfs, low-redshift quasars, and spiral and elliptical galaxies. We point outthe high degree of correlation between the UV color and the contribution of theUV flux to the UV-optical-IR flux of galaxies detected by GALEX.