The Complete Calibration of the Color-Redshift Relation (C3R2) Survey:
  Analysis and Data Release 2
  The Complete Calibration of the Color-Redshift Relation (C3R2) survey is a
multi-institution, multi-instrument survey that aims to map the empirical
relation of galaxy color to redshift to i~24.5 (AB), thereby providing a firm
foundation for weak lensing cosmology with the Stage IV dark energy missions
Euclid and WFIRST. Here we present 3171 new spectroscopic redshifts obtained in
the 2016B and 2017A semesters with a combination of DEIMOS, LRIS, and MOSFIRE
on the Keck telescopes. The observations come from all of the Keck partners:
Caltech, NASA, the University of Hawaii, and the University of California.
Combined with the 1283 redshifts published in DR1, the C3R2 survey has now
obtained and published 4454 high quality galaxy redshifts. We discuss updates
to the survey design and provide a catalog of photometric and spectroscopic
data. Initial tests of the calibration method performance are given, indicating
that the sample, once completed and combined with extensive data collected by
other spectroscopic surveys, should allow us to meet the cosmology requirements
for Euclid, and make significant headway toward solving the problem for WFIRST.
We use the full spectroscopic sample to demonstrate that galaxy brightness is
weakly correlated with redshift once a galaxy is localized in the Euclid or
WFIRST color space, with potentially important implications for the
spectroscopy needed to calibrate redshifts for faint WFIRST and LSST sources.
