Variability selected high-redshift quasars on SDSS Stripe 82

¹ CEA, Centre de Saclay, Irfu/SPP, 91191 Gif-sur-Yvette, France
e-mail: nathalie.palanque-delabrouille@cea.fr
² Department of Astronomy, University of Illinois at Urbana-Champaign, Urbana IL 61801, USA
³ Université Paris 6, Institut d’Astrophysique de Paris, CNRS UMR7095, 98bis Boulevard Arago, 75014 Paris, France
⁴ Lawrence Berkeley National Lab, 1 Cyclotron Road, Berkeley, CA 94720, USA
⁵ Brookhaven National Laboratory, Bldg 510, Upton, NY 11973, USA
⁶ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
⁷ APC, 10 rue Alice Domon et Léonie Duquet, 75205 Paris Cedex 13, France
⁸ Center for Cosmology and Particle Physics, New York University, New York, NY 10003, USA
⁹ University of Utah, Dept. of Physics & Astronomy, 115 S 1400 E, Salt Lake City, UT 84112, USA
¹⁰ Department of Astronomy and Astrophysics, The Pennsylvania State University, 525 Davey Laboratory, University Park, PA 16802, USA

Received: 2 December 2010
Accepted: 6 April 2011

Abstract

The SDSS-III BOSS Quasar survey will attempt to observe z > 2.15 quasars at a density of at least 15 per square degree to yield the first measurement of the baryon acoustic oscillations in the Ly-α forest. To help reaching this goal, we have developed a method to identify quasars based on their variability in the ugriz optical bands. The method has been applied to the selection of quasar targets in the SDSS region known as Stripe 82 (the southern equatorial stripe), where numerous photometric observations are available over a 10-year baseline. This area was observed by BOSS during September and October 2010. Only 8% of the objects selected via variability are not quasars, while 90% of the previously identified high-redshift quasar population is recovered. The method allows for a significant increase in the z > 2.15 quasar density over previous strategies based on optical (ugriz) colors, achieving a density of 24.0 deg^-2 on average down to g ~ 22 over the 220 deg² area of Stripe 82. We applied this method to simulated data from the Palomar Transient Factory and from Pan-STARRS, and showed that even with data that have sparser time sampling than what is available in Stripe 82, including variability in future quasar selection strategies would lead to increased target selection efficiency in the z > 2.15 redshift range. We also found that broad absorption line quasars are preferentially present in a variability than in a color selection.