Euclid Preparation: XIV. The Complete Calibration of the Color-Redshift
  Relation (C3R2) Survey: Data Release 3
  The Complete Calibration of the Color-Redshift Relation (C3R2) survey is
obtaining spectroscopic redshifts in order to map the relation between galaxy
color and redshift to a depth of i ~ 24.5 (AB). The primary goal is to enable
sufficiently accurate photometric redshifts for Stage IV dark energy projects,
particularly Euclid and the Roman Space Telescope, which are designed to
constrain cosmological parameters through weak lensing. We present 676 new
high-confidence spectroscopic redshifts obtained by the C3R2 survey in the
2017B-2019B semesters using the DEIMOS, LRIS, and MOSFIRE multi-object
spectrographs on the Keck telescopes. Combined with the 4454 redshifts
previously published by this project, the C3R2 survey has now obtained and
published 5130 high-quality galaxy spectra and redshifts. If we restrict
consideration to only the 0.2 < z(phot) < 2.6 range of interest for the Euclid
cosmological goals, then with the current data release C3R2 has increased the
spectroscopic redshift coverage of the Euclid color space from 51% (as reported
by Masters et al. 2015) to the current 91%. Once completed and combined with
extensive data collected by other spectroscopic surveys, C3R2 should provide
the spectroscopic calibration set needed to enable photometric redshifts to
meet the cosmology requirements for Euclid, and make significant headway toward
solving the problem for Roman.
