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Published August 20, 2012 | Published
Journal Article Open

Evolution of the Quasar Luminosity Function over 3 < z < 5 in the COSMOS Survey Field


We investigate the high-redshift quasar luminosity function (QLF) down to an apparent magnitude of I_AB = 25 in the Cosmic Evolution Survey (COSMOS). Careful analysis of the extensive COSMOS photometry and imaging data allows us to identify and remove stellar and low-redshift contaminants, enabling a selection that is nearly complete for type-1 quasars at the redshifts of interest. We find 155 likely quasars at z > 3.1, 39 of which have prior spectroscopic confirmation. We present our sample in detail and use these confirmed and likely quasars to compute the rest-frame UV QLF in the redshift bins 3.1 < z < 3.5 and 3.5 < z < 5. The space density of faint quasars decreases by roughly a factor of four from z ~ 3.2 to z ~ 4, with faint-end slopes of β ~ –1.7 at both redshifts. The decline in space density of faint optical quasars at z > 3 is similar to what has been found for more luminous optical and X-ray quasars. We compare the rest-frame UV luminosity functions found here with the X-ray luminosity function at z > 3, and find that they evolve similarly between z ~ 3.2 and z ~ 4; however, the different normalizations imply that roughly 75% of X-ray bright active galactic nuclei (AGNs) at z ~ 3-4 are optically obscured. This fraction is higher than found at lower redshift and may imply that the obscured, type-2 fraction continues to increase with redshift at least to z ~ 4. Finally, the implications of the results derived here for the contribution of quasars to cosmic reionization are discussed.

Additional Information

© 2012 American Astronomical Society. Received 2011 November 29; accepted 2012 July 6; published 2012 August 7. We gratefully acknowledge the contributions of the entire COSMOS collaboration. More information about the COSMOS survey is available at http://www.astro.caltech.edu/cosmos. We acknowledge the anonymous referee for suggestions that significantly improved this work. We thank Dr. Eilat Glikman for helpful discussions. M.S. and G.H. acknowledge support by the German Deutsche Forschungsgemeinschaft, DFG Leibniz Prize (FKZ HA 1850/28-1). D.M. gratefully acknowledges the support of the Infrared Processing and Analysis Center (IPAC) Visiting Graduate Fellowship program, as well as the Carnegie Visiting Graduate Fellowship program.

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