Evolution of the Quasar Luminosity Function over 3 < z < 5 in the COSMOS Survey Field
Abstract
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.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- August 2012
- DOI:
- 10.1088/0004-637X/755/2/169
- arXiv:
- arXiv:1207.2154
- Bibcode:
- 2012ApJ...755..169M
- Keywords:
-
- cosmology: observations;
- galaxies: luminosity function;
- mass function;
- Galaxy: evolution;
- quasars: general;
- Astrophysics - Cosmology and Nongalactic Astrophysics
- E-Print:
- Accepted for publication in ApJ. 17 pages, 14 figures