Exploring z>0.6 LRGs from SDSS and WISE


Photo of Jeffrey Newman
Jeffrey Newman
University of Pittsburgh


A spectroscopic survey of distant (z > 0.6) luminous red galaxies (LRGs) selected using WISE photometry, with the goal of better calibrating photometric redshifts of these galaxies, and therefore improving distance estimates

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
HIZ_LRG 21 A candidate z > 0.6 LRG, selected using a narrower r-i color constraint and observed at higher priority 8,291
LRG_ROUND3 22 A candidate z > 0.6 LRG, selected using a broader r-i color constraint and observed at lower priority 2,543


This ancillary science program used WISE and SDSS photometry to identify a sample of luminous red galaxies (LRGs) at redshift z > 0.6. The search for these galaxies takes advantage of the fact that old stellar populations show a local maximum in their spectral energy distributions at 1.6 µm (corresponding to a local minimum in the opacity of the H- ion) — and at z ~ 0.6, this “1.6 µm bump” is redshifted into the WISE W1 band.

The resulting sample can be used to calibrate photometric redshifts in this range, and to test target selection techniques for the SDSS’s upcoming eBOSS survey.

Target Selection

Targets for this program were divided into a higher priority sample indicated by the target flag HIZ_LRG and a lower priority sample indicated by LRG_ROUND3. For both samples, objects were required to have

(imodel < 20.0 || zmodel < 20.0) && (zfib2 < 21.7 || ifib2 < 22.0)

Objects in the HIZ_LRG sample were selected to have

(r – i) > 0.98 && (r – W1) > 2(r – i) – 0.5

The LRG_ROUND3 sample used the same r – W1 cut, but the (r – i) color cut was bluer, with (r – i) > 0.85, to explore a broader range of galaxy colors.