The extended Baryon Oscillation Spectroscopic Survey: Variability selection and quasar luminosity function

¹ CEA, Centre de Saclay, Irfu/SPP, 91 191 Gif-sur-Yvette, France
e-mail: nathalie.palanque-delabrouille@cea.fr
² INAF−Osservatorio Astronomico di Trieste, via G. B. Tiepolo 11, 34131 Trieste, Italy
³ UPMC-CNRS, UMR7095, Institut d’Astrophysique de Paris, 75014, Paris, France
⁴ Department of Physics and Astronomy, University of Utah, Salt Lake City, UT 84112, USA
⁵ Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85 721, USA
⁶ Department of Physics and Astronomy, University of Wyoming, Laramie, WY 82 071, USA
⁷ Department of Astronomy and Space Science, Sejong University, 143-747 Seoul, Korea
⁸ Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, CA94 720, USA
⁹ Department of Astronomy and Astrophysics, 525 Davey Laboratory, The Pennsylvania State University, University Park, PA 16 802, USA
¹⁰ Institute for Gravitation and the Cosmos, The Pennsylvania State University, University Park, PA 16802, USA
¹¹ Instituto de Astrofisica de Canarias (IAC), 38200 La Laguna, Tenerife, Spain
¹² Universidad de La Laguna (ULL), Dept. Astrofisica, 38206 La Laguna, Tenerife, Spain
¹³ Center for Cosmology and Particle Physics, New York University, New York, NY 10 003, USA

Received: 17 September 2015
Accepted: 8 December 2015

Abstract

The extended Baryon Oscillation Spectroscopic Survey of the Sloan Digital Sky Survey (SDSS-IV/eBOSS) has an extensive quasar program that combines several selection methods. Among these, the photometric variability technique provides highly uniform samples, which are unaffected by the redshift bias of traditional optical-color selections, when z = 2.7−3.5 quasars cross the stellar locus or when host galaxy light affects quasar colors at z< 0.9. We present the variability selection of quasars in eBOSS, focusing on a specific program that led to a sample of 13 876 quasars to g_dered = 22.5 over a 94.5 deg² region in Stripe 82, which has an areal density 1.5 times higher than over the rest of the eBOSS footprint. We use these variability-selected data to provide a new measurement of the quasar luminosity function (QLF) in the redshift range of 0.68 <z< 4.0. Our sample is denser and reaches more deeply than those used in previous studies of the QLF, and it is among the largest ones. At the faint end, our QLF extends to M_g(z = 2) = −21.80 at low redshift and to M_g(z = 2) = −26.20 at z ~ 4. We fit the QLF using two independent double-power-law models with ten free parameters each. The first model is a pure luminosity-function evolution (PLE) with bright-end and faint-end slopes allowed to be different on either side of z = 2.2. The other is a simple PLE at z< 2.2, combined with a model that comprises both luminosity and density evolution (LEDE) at z> 2.2. Both models are constrained to be continuous at z = 2.2. They present a flattening of the bright-end slope at high redshift. The LEDE model indicates a reduction of the break density with increasing redshift, but the evolution of the break magnitude depends on the parameterization. The models are in excellent accord, predicting quasar counts that agree within 0.3% (resp., 1.1%) to g< 22.5 (resp., g< 23). The models are also in good agreement over the entire redshift range with models from previous studies.