arXiv:1212.4493v2 [astro-ph.CO] 22 Apr 2013 Draft version June 13, 2018 Preprint typeset using L A T E X style emulateapj v. 5/2/11 THE Z =5 QUASAR LUMINOSITY FUNCTION FROM SDSS STRIPE 82 OBSERVATIONS REPORTED HERE WERE OBTAINED AT THE MMT OBSERVATORY, A JOINT FACILITY OF THE SMITHSONIAN INSTITUTION AND THE UNIVERSITY OF ARIZONA. THIS PAPER ALSO INCLUDES DATA GATHERED WITH THE 6.5-M MAGELLAN TELESCOPES LOCATED AT LAS CAMPANAS OBSERVATORY, CHILE. Ian D. McGreer 1 , Linhua Jiang 2, † , Xiaohui Fan 1 , Gordon T. Richards 3 , Michael A. Strauss 4 , Nicholas P. Ross 5 , Martin White 5,6 , Yue Shen 7 , Donald P. Schneider 8,9 , Adam D. Myers 10 , W. Niel Brandt 8 , Colin DeGraf 11 , Eilat Glikman 12 , Jian Ge 13 , Alina Streblyanska 14,15 Draft version June 13, 2018 ABSTRACT We present a measurement of the Type I quasar luminosity function at z = 5 using a large sample of spectroscopically confirmed quasars selected from optical imaging data. We measure the bright end (M 1450 < −26) with Sloan Digital Sky Survey (SDSS) data covering ∼ 6000 deg 2 , then extend to lower luminosities (M 1450 < −24) with newly discovered, faint z ∼ 5 quasars selected from 235 deg 2 of deep, coadded imaging in the SDSS Stripe 82 region (the celestial equator in the Southern Galactic Cap). The faint sample includes 14 quasars with spectra obtained as ancillary science targets in the SDSS-III Baryon Oscillation Spectroscopic Survey (BOSS), and 59 quasars observed at the MMT and Magellan telescopes. We construct a well-defined sample of 4.7 <z< 5.1 quasars that is highly complete, with 73 spectroscopic identifications out of 92 candidates. Our color selection method is also highly efficient: of the 73 spectra obtained, 71 are high redshift quasars. These observations reach below the break in the luminosity function (M ∗ 1450 ≈−27). The bright end slope is steep (β  −4), with a constraint of β< −3.1 at 95% confidence. The break luminosity appears to evolve strongly at high redshift, providing an explanation for the flattening of the bright end slope reported previously. We find a factor of ∼ 2 greater decrease in the number density of luminous quasars (M 1450 < −26) from z = 5 to z = 6 than from z = 4 to z = 5, suggesting a more rapid decline in quasar activity at high redshift than found in previous surveys. Our model for the quasar luminosity function predicts that quasars generate ∼ 30% of the ionizing photons required to keep hydrogen in the universe ionized at z = 5. Subject headings: quasars: general imcgreer@as.arizona.edu 1 Steward Observatory, The University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721–0065, USA 2 School of Earth and Space Exploration, Arizona State Uni- versity, Tempe, AZ 85287, USA 3 Department of Physics, Drexel University, 3141 Chestnut Street, Philadelphia, PA 19104, USA 4 Department of Astrophysical Sciences, Princeton University, Princeton, NJ 08544, USA 5 Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, CA 92420, USA 6 Department of Physics, 366 LeConte Hall, University of Cal- ifornia, Berkeley, CA 94720, USA 7 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA 8 Department of Astronomy and Astrophysics, The Pennsyl- vania State University, 525 Davey Laboratory, University Park, PA 16802, USA 9 Institute for Gravitation and the Cosmos, The Pennsylvania State University, University Park, PA 16802, USA 10 Department of Physics and Astronomy, University of Wyoming, Laramie, WY 82071, USA 11 McWilliams Center for Cosmology, Carnegie Mellon Uni- versity, 5000 Forbes Avenue, Pittsburgh, PA 15213, U.S.A. 12 Department of Physics and Yale Center for Astronomy and Astrophysics, Yale University, P.O. Box 208121, New Haven, CT 06520-8121 13 Dept. of Astronomy, University of Florida, 211 Bryant Space Science Center, Gainesville, FL, 32611, USA 14 Instituto de Astrofisica de Canarias (IAC), E-38200 La La- guna, Tenerife, Spain 15 Dept. Astrofisica, Universidad de La Laguna (ULL), E- 38206 La Laguna, Tenerife, Spain † Hubble Fellow