High-Redshift Quasars from SDSS and UKIDSS


Ian McGreer
University of Arizona


Spectra of candidate high-redshift quasars seen by both the SDSS and the UKIRT Infrared Deep Sky Survey

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
HIZQSO82 0 Quasar candidate in SDSS stripe 82 selected with UKIDSS YJHK photometry using a more relaxed color cut 66 62
HIZQSOIR 1 Quasar candidate selected with UKIDSS YJHK photometry 121 109


This ancillary program targets high-redshift quasar candidates through a combination of color cuts combining SDSS ugriz PSF magnitudes and YJHK aperture photometry (with 1″ radius apertures) from the UKIRT Infrared Deep-Sky Survey.

The addition of infrared photometry from UKIDSS provides leverage to separate quasars at z ~ 5.5 from the red end of the stellar locus. (In fact, the SDSS quasar survey was limited to z < 5.4 by the strong overlap in ugriz colors between high-redshift quasars and red stars.)

Target Selection

Quasar candidates were selected by matching stellar objects from the SDSS DR7 and UKIDSS DR3 databases. The initial target selection sample was drawn from SDSS with color cuts r – i > 1.4 and i – z > 0.5, as well as a magnitude cut of z < 20.2.

Likely stars were rejected with the criteria (H – K)Vega < 0.53 or (J – KVega) < 1.3(Y – JVega) + 0.32, taking advantage of the fact that quasars are redder than M stars at longer wavelengths and bluer at shorter wavelengths. The remaining candidates are then prioritized based on their izYJK colors. Objects selected using these criteria are given the HIZQSOIR target bit.

In Stripe 82, the color criteria used for prioritizing targets were slightly relaxed, owing to the coadded ugriz photometry available from Annis et al. (2011), which greatly reduced initial stellar contamination from riz selection. Objects selected with the relaxed criteria on Stripe 82 are given the HIZQSO82 target bit.


Annis, J.A., Soares-Santos, M., Strauss, M.A., Becker, A.C., Dodelson, S., Fan, X., Gunn, J.E., Hao, J., Ivezić, Ž, Jester, S., Jiang, L., Johnston, D.E., Kubo, J.M., Lampeitl, H., Lin, H., Lupton, R.H., Miknaitis, G., Seo, H-J., Simet, M., & Yanny, B., 2011, arXiv:1111.6619