NEAR-INFRARED PHOTOMETRIC PROPERTIES OF 130,000 QUASARS: AN SDSS-UKIDSS-MATCHED CATALOG

Abstract

We present a catalog of over 130,000 quasar candidates with near-infrared (NIR) photometric properties, with an areal coverage of approximately 1200 deg2. This is achieved by matching the Sloan Digital Sky Survey (SDSS) in the optical ugriz bands to the UKIRT Infrared Digital Sky Survey (UKIDSS) Large Area Survey (LAS) in the NIR YJHK bands. We match the ≈1 million SDSS DR6 Photometric Quasar catalog to Data Release 3 of the UKIDSS LAS (ULAS) and produce a catalog with 130,827 objects with detections in one or more NIR bands, of which 74,351 objects have optical and K-band detections and 42,133 objects have the full nine-band photometry. The majority (∼85%) of the SDSS objects were not matched simply because these were not covered by the ULAS. The positional standard deviation of the SDSS Quasar to ULAS matches is δR.A. = 01370 and δdecl. = 01314. We find an absolute systematic astrometric offset between the SDSS Quasar catalog and the UKIDSS LAS, of |R.A.offset| = 0025 and |decl.offset| = 0040; we suggest the nature of this offset to be due to the matching of catalog, rather than image, level data. Our matched catalog has a surface density of ≈53 deg−2 for K ⩽ 18.27 objects; tests using our matched catalog, along with data from the UKIDSS Deep Extragalactic Survey, imply that our limiting magnitude is i ≈ 20.6. Color–redshift diagrams, for the optical and NIR, show a close agreement between our matched catalog and recent quasar color models at redshift z ≲ 2.0, while at higher redshifts, the models generally appear to be bluer than the mean observed quasar colors. The gJK and giK color spaces are used to examine methods of differentiating between stars and (mid-redshift) quasars, the key to currently ongoing quasar surveys. Finally, we report on the NIR photometric properties of high, z > 4.6, and very high, z > 5.7, redshift previously discovered quasars.