GALEXFAR-ULTRAVIOLET COLOR SELECTION OF UV-BRIGHT HIGH-REDSHIFT QUASARS

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Abstract
We study the small population of high-redshift (z(em) > 2.7) quasars detected by the Galaxy Evolution Explorer(GALEX), whose far-UV emission is not extinguished by intervening HI Lyman limit systems. These quasars are of particular importance to detect intergalactic He II absorption along their sight lines. We correlate almost all verified z(em) > 2.7 quasars to the GALEX GR4 source catalog covering similar to 25,000 deg(2), yielding 304 sources detected at signal-to-noise ratio (S/N) > 3. However, similar to 50% of these are only detected in the GALEX NUV band, signaling the truncation of the FUV flux by low-redshift optically thick Lyman limit systems. We exploit the GALEX UV color m(FUV) - m(NUV) to cull the most promising targets for follow-up studies, with blue (red) GALEX colors indicating transparent (opaque) sight lines. Extensive Monte Carlo simulations indicate an He II detection rate of similar to 60% for quasars with m(FUV) - m(NUV) less than or similar to 1 at z(em) less than or similar to 3.5, a similar to 50% increase over GALEX searches that do not include color information. We regard 52 quasars detected at S/N > 3 to be most promising for Hubble Space Telescope follow-up, with an additional 114 quasars if we consider S/N > 2 detections in the FUV. Combining the statistical properties of HI absorbers with the Sloan Digital Sky Survey (SDSS) quasar luminosity function, we predict a large all-sky population of similar to 200 quasars with z(em) > 2.7 and i less than or similar to 19 that should be detectable at the He II edge at m(304) < 21. However, SDSS provides just half of the NUV-bright quasars that should have been detected by SDSS and GALEX. With mock quasar photometry we revise the SDSS quasar selection function, finding that SDSS systematically misses quasars with blue u - g less than or similar to 2 colors at 3 less than or similar to z(em) less than or similar to 3.5 due to overlap with the stellar locus in color space. Our color-dependent SDSS selection function naturally explains the inhomogeneous u - g color distribution of SDSS DR7 quasars as a function of redshift and the color difference between color-selected and radio-selected SDSS quasars. Moreover, it yields excellent agreement between the observed and the predicted number of GALEX UV-bright SDSS quasars. We confirm our previous claims that SDSS preferentially selects 3 less than or similar to z(em) less than or similar to 3.5 quasars with intervening HI Lyman limit systems. Our results imply that broadband optical color surveys for 3 less than or similar to z(em) less than or similar to 3.5 quasars have likely underestimated their space density by selecting intergalactic medium sight lines with an excess of strong HI absorbers.