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High-Redshift Quasars


Spectra of candidate high-redshift quasars identified from SDSS imaging. Separate target selection was carried out in the 220 deg2 footprint of SDSS Stripe 82 and in the 7,650 deg2 area of DR7 imaging.

Finding Targets

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

(bit name)
Bit number Target Description Target density
QSO_GRI 29 Quasar candidate selected in gri color space 2.7 (Stripe 82);
0.8 (elsewhere)
QSO_HIZ 30 Quasar candidate detected only in SDSS i and z filters 0.7 (Stripe 82);
0.2 (elsewhere)
QSO_RIZ 31 Quasar candidate selected in riz color space 1.2 (Stripe 82);
0.6 (elsewhere)


High-redshift quasars trace the evolution of early generations of supermassive black holes, provide tests for models of quasar formation and AGN evolution, and probe evolution in the intergalactic medium (IGM). However, the BOSS quasar survey (Ross et al. 2012) selects objects only to z ~ 3.5. Light emitted by high-redshift quasars at wavelengths shorter than Lyα is absorbed by the intergalactic medium, meaning that for redshifts z > 5.7, quasars are detected in only the z band, the reddest filter in the SDSS imaging survey.

This BOSS ancillary target program uses areas of SDSS imaging where there is overlap between stripes, meaning that objects in those areas have more than one SDSS detection. This serves to reduce contamination from cosmic rays and improves the photometry, allowing selection of high-redshift quasar candidates in three redshift ranges to fainter magnitudes than in the SDSS survey.

Primary contact

Xiaohui Fan
University of Arizona
fan -at-

Other contacts

Linhua Jiang, Gordon Richards, Michael Strauss

Target Selection Details

For the main survey, PSF magnitudes for objects with multiple detections were extracted from the Neighbors table in DR7 SkyServer, and detections were co-added in each band. Target selection was performed on the coadded photometry.

For objects within Stripe 82, PSF magnitudes were extracted from the coadded image catalogs described in Annis et al. (2011). The Stripe 82 coadd catalogs combine roughly twenty epochs instead of just two, and thus allow selection of targets at fainter magnitudes.

The first part of this ancillary target program targets objects with color cuts similar to those imposed in the SDSS-I quasar survey (Richards et al. 2002). The SDSS quasar target selection defined two inclusion regions in gri and riz color space for targeting quasars at z > 3.6 and z > 4.5, respectively (Richards et al. 2002). Both this ancillary target selection program and the High-Redshift Quasars from SDSS and UKIDSS program are straightforward extensions of these color selection criteria to fainter magnitudes, with limits of iPSF < 21.3 in the main survey regions and iPSF < 21.5 (compared to iPSF < 20.2 for SDSS-I). The ancillary target flag QSO_GRI is assigned to objects that met the gri color criteria, and the flag QSO_RIZ to those meeting the riz criteria. In both cases, the primary color cut follows a diagonal line in the respective color plane.


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
Richards, G. T., et al. 2002, AJ, 123, 2945
Ross, N. P., et al. 2012, ApJS, 199, 3