K-Band Selected Sample of Quasars


Photo of Richard McMahon
Richard McMahon
University of Cambridge


Quasar candidates in the 220-square-degree footprint of Stripe 82 that were selected by matching SDSS and UKIDSS photometry

Finding Targets

An object whose ANCILLARY_TARGET2 value includes one or more of the bitmasks in the following table was targeted for spectroscopy as part of this ancillary target program. See SDSS bitmasks to learn how to use these values to identify objects in this ancillary target program.

Program (bit name) Bit number Target Description Number of Fibers Number of Unique Primary Objects
KQSO_BOSS 2 Quasars selected from UKIDSS photometry 192 178


Potential quasars were identified by photometering data from the UKIRT Infrared Deep Sky Survey (UKIDSS) at the position of SDSS sources to a limiting magnitude KAB < 19.0 (KVega < 17.1). The goal of this ancillary target program was to identify highly red and reddened quasars that would have been missed by the main BOSS Quasar Target Selection algorithms. The sample described here allows investigations of, for example, the incidence of broad absorption line (BAL) quasars, or of highly reddened gravitationally-lensed quasars.

This ancillary target program superseded the Reddened Quasars target selection program, since the superior UKIDSS photometry could target red, reddened, and high-redshift quasars better than 2MASS photometry. In addition, the color cut of (uPSF – gPSF) > 0.4 used here better complemented the BOSS mission to target mostly higher-redshift quasars.

The targets not included in Stripe 82 were effectively incorporated into the BONUS sample through the XDQSO target selection (Bovy et al. 2011, Bovy et al. 2012); therefore, this ancillary program was not continued for the remainder of the BOSS footprint.

Target Selection

In the original selection, approximately 2,000 targets were identified that have

(gPSF – iPSF) < 1.153 * (iPSF – K) – 1.401

to avoid the color space locus of the stellar main sequence, and (uPSF – gPSF) > 0.4 to exclude UV-excess quasars (i.e., unreddened quasars at redshifts of about z > 2.15).


Bovy, J., et al. 2011, ApJ, 729, 141, doi:10.1088/0004-637X/729/2/141

Bovy, J., et al. 2012, ApJ, 749, 41, doi:10.1088/0004-637X/749/1/41