Southern cosmology survey. III. QSOs from combined GALEX and optical photometry

We present catalogs of QSO candidates selected using photometry from Galaxy Evolution Explorer (GALEX) combined with the Sloan Digital Sky Survey (SDSS) in the Stripe 82 region and Blanco Cosmology Survey (BCS) near declination -55°. The SDSS region contains ∼700 objects with magnitude i < 20 and ≳3600 objects with i < 21.5 in a ≳60 deg² sky region, while the BCS region contains ≳280 objects with magnitude i < 20 and ∼2000 objects with i < 21.5 for a 11 deg² sky region that is being observed by three current microwave Sunyaev-Zeldovich surveys. Our QSO catalog is the first one in the BCS region. Deep GALEX exposures (≳2000 s in Fuv and N_UV, except in three fields) provide high signal-to-noise photometry in the GALEX bands (F_UVw, N_UV < 24.5 mag). From this data, we select QSO candidates using only GALEX and optical r-band photometry, using the method given by Atlee & Gould. In the Stripe 82 field, 60% (30%) of the GALEX-selected QSOs with optical magnitude i < 20 (i < 21.5) also appear in the Richards et al. QSO catalog constructed using five-band optical SDSS photometry. Comparison with the same catalog by Richards et al. shows that the completeness of the sample is approximately 40% (25%). However, for regions of the sky with very low dust extinction, like the BCS 23-hr field and the Stripe 82 between 0° and 10° in R.A., our completeness is close to 95%, demonstrating that deep GALEX observations are almost as efficient as multiwavelength observations at finding QSOs. GALEX observations thus provide a viable alternate route to QSO catalogs in sky regions where u-band optical photometry is not available. The full catalog is available at http://www.ice.csic.es/personal/jimenez/PHOTOZ.