Efficient Photometric Selection of Quasars From the Sloan Digital Sky Survey. Ii. ∼1, 000, 000 Quasars From Data Release 6

Abstract

We present a catalog of 1,172,157 quasar candidates selected from the photometric imaging data of the Sloan Digital Sky Survey (SDSS). The objects are all point sources to a limiting magnitude of i = 21.3 from 8417 deg 2 of imaging from SDSS Data Release 6 (DR6). This sample extends our previous catalog by using the latest SDSS public release data and probing both UV-excess and high-redshift quasars. While the addition of high-redshift candidates reduces the overall efficiency (quasars:quasar candidates) of the catalog to ∼ 80%, it is expected to contain no fewer than 850,000 bona fide quasars -∼ 8 times the number of our previous sample, and ∼ 10 times the size of the largest spectroscopic quasar catalog. Cross-matching between our photometric catalog and spectroscopic quasar catalogs from both the SDSS and 2dF Surveys, yields 88,879 -2spectroscopically confirmed quasars. For judicious selection of the most robust UV-excess sources (∼ 500, 000 objects in all), the efficiency is nearly 97%more than sufficient for detailed statistical analyses. The catalog's completeness to type 1 (broad-line) quasars is expected to be no worse than 70%, with most missing objects occurring at z < 0.7 and 2.5 < z < 3.0. In addition to classification information, we provide photometric redshift estimates (typically good to ∆z ± 0.3 [2σ]) and cross-matching with radio, X-ray, and proper motion catalogs. Finally, we consider the catalog's utility for determining the optical luminosity function of quasars and are able to confirm the flattening of the bright-end slope of the quasar luminosity function at z ∼ 4 as compared to z ∼ 2.

Clustering of intermediate redshift quasars using the final SDSS III-BOSS sample

“…As such, a heterogeneous range of multi-wavelength data was used to target BOSS quasars, and a number of methods were incorporated into the survey in addition to XDQSO (Richards et al 2009;Yèche et al 2010;Kirkpatrick et al 2011;see Ross et al 2012b). This heterogeneous or "BONUS" approach augmented the CORE sample.…”

Clustering on very small scales from a large sample of confirmed quasar pairs: does quasar clustering track from Mpc to kpc scales?

“…Our starting sample consists of 1,172,157 high-probability candidate quasars identified by Richards et al (2009) using Kernel Density Estimation (henceforth KDE; see also Richards et al 2004). Richards et al (2009) applied the KDE technique to a test sample consisting of all point sources in SDSS Data Release 6 (DR6;Adelman-McCarthy et al 2008) imaging down to a limiting magnitude of i = 21.3.…”

“…Richards et al (2009) applied the KDE technique to a test sample consisting of all point sources in SDSS Data Release 6 (DR6;Adelman-McCarthy et al 2008) imaging down to a limiting magnitude of i = 21.3. These test data were labeled as a "star" or a "quasar" using a non-parametric Bayesian classifier, based on their position in ugriz colour space.…”

“…Although the search for high redshift quasar pairs dates back to individual discoveries of quasars at z ∼ 4 (e.g., Crampton et al 1988;McCarthy et al 1988;Meylan et al 1990;Djorgovski 1991;Schneider, Schmidt & Gunn 1994;Hewett et al 1998;Zhdanov & Surdej 2001), with the development of photometric selection algorithms to build homogeneous sets of quasar candidates from large imaging surveys (e.g., Richards et al 2004Richards et al , 2009, it became possible to conduct more homogeneous searches by prioritizing highly-probable close quasar pairs and following them up with long-slit spectroscopic surveys (e.g., Hennawi et al 2006b;Myers et al 2007bMyers et al , 2008. These surveys focused on quasar pairs separated by less than 2000 km s −1 in redshift-space in order to measure small-scale clustering, denoting such pairs "binary quasars," a term that has appeared for decades in the literature (e.g.…”

“…We identify high-probability candidate close quasar pairs from a homogeneous catalogue of candidates and follow them up with confirming spectroscopy. Our target sample is drawn from quasar candidates selected using Kernel Density Estimation (KDE) by Richards et al (2009). This "KDE" sample is not only large, it is pure 2 , so presents an efficient parent sample to mine for binary quasars.…”