GALEX far-UV color selection of UV-bright high-redshift quasars

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  • 20 April 2010
Abstract
We study the small population of high-redshift z>2.7 quasars detected by GALEX, whose far-UV emission is not extinguished by intervening HI Lyman limit systems. These quasars are of particular importance to detect intergalactic HeII absorption along their sightlines. We correlate verified z>2.7 quasars to the GALEX GR4 source catalog, yielding 803 sources. However, ~70% of these are only detected in the GALEX NUV band, signaling the truncation of the FUV flux by low-redshift Lyman limit systems. We exploit the GALEX UV color to cull the most promising targets for follow-up studies, with blue (red) colors indicating transparent (opaque) sightlines. Extensive Monte Carlo simulations indicate a HeII detection rate of ~60% for quasars with m_FUV-m_NUV2.7 and i<19 should be detectable at the HeII edge at m_304<21. However, SDSS provides just half of the NUV-bright quasars that should have been detected by SDSS & GALEX. We revise the SDSS quasar selection function, finding that SDSS systematically misses quasars with blue u-g<2 colors at 3<z<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 quasars with 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<z<3.5 quasars with intervening HI Lyman limit systems. Our results imply that broadband optical color surveys for 3<z<3.5 quasars have likely underestimated their space density by selecting IGM sightlines with an excess of strong HI absorbers.

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