NED Abstract 
Copyright by American Astronomical Society.
Reproduced by permission

2005AJ....130.1022A

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

Marcel A. Agueros, Zeljko Ivezic, Kevin R. Covey, Mirela Obric, Lei Hao,
Lucianne M. Walkowicz, Andrew A. West, Daniel E. Vanden Berk, Robert H.
Lupton, Gillian R. Knapp, James E. Gunn, Gordon T. Richards, John
Bochanski, Jr., Alyson Brooks, Mark Claire, Daryl Haggard, Nathan Kaib, Amy
Kimball, Stephanie M. Gogarten, Anil Seth, and Michael Solontoi 

Abstract. We discuss the ultraviolet, optical, and infrared properties of
the Sloan Digital Sky Survey (SDSS) sources detected by the Galaxy
Evolution Explorer (GALEX) as part of its All-sky Imaging Survey Early
Release Observations. Virtually all (>99%) the GALEX sources in the overlap
region are detected by SDSS; those without an SDSS counterpart within our
6" search radius are mostly unflagged GALEX artifacts. GALEX sources
represent ~2.5% of all SDSS sources within these fields, and about half are
optically unresolved. Most unresolved GALEX-SDSS sources are bright (r<18
mag), blue, turnoff, thick-disk stars and are typically detected only in
the GALEX near-ultraviolet (NUV) band. The remaining unresolved sources
include low-redshift quasars (z<2.2), white dwarfs, and white dwarf -- M
dwarf pairs, and these dominate the optically unresolved sources detected
in both GALEX bands.Almost all the resolved SDSS sources detected by GALEX
are fainter than the SDSS main spectroscopic limit. (Conversely, of the
SDSS galaxies in the main spectroscopic sample, about 40% are detected in
at least one GALEX band.) These sources have colors consistent with those
of blue (spiral) galaxies (u-r<2.2), and most are detected in both GALEX
bands. Measurements of their UV colors allow much more accurate and robust
estimates of star formation history than are possible using only SDSS data.
Indeed, galaxies with the most recent (<~20 Myr) star formation can be
robustly selected from the GALEX data by requiring that they be brighter in
the far-ultraviolet (FUV) than in the NUV band. However, older starburst
galaxies have UV colors similar to those of active galactic nuclei and thus
cannot be selected unambiguously on the basis of GALEX fluxes alone.
Additional information, such as spatially resolved FUV emission, optical
morphology, or X-ray and radio data, is needed before blue GALEX colors can
be unambiguously interpreted as a sign of recent star formation.With the
aid of Two Micron All Sky Survey data, we construct and discuss median
10-band UV through infrared spectral energy distributions for turnoff
stars, hot white dwarfs, low-redshift quasars, and spiral and elliptical
galaxies. We point out the high degree of correlation between the UV color
and the contribution of the UV flux to the UV through infrared flux of
galaxies detected by GALEX; for example, this correlation can be used to
predict the SDSS z-band measurement, using only two GALEX fluxes, with a
scatter of only 0.7 mag. 

Keywords: Catalogs, Galaxies: Active, Galaxies: Starburst, Ultraviolet:
Galaxies, Ultraviolet: General, Ultraviolet: Stars