# The Complete Calibration of the Color-Redshift Relation (C3R2) Survey: Analysis and Data Release 2

## TL;DR

The C3R2 survey provides a comprehensive set of galaxy redshifts to calibrate the color-redshift relation, crucial for weak lensing cosmology with upcoming dark energy missions Euclid and WFIRST.

## Contribution

This paper presents the second data release of the C3R2 survey, including 3171 new spectroscopic redshifts, updates to survey design, and analysis demonstrating its potential to meet cosmological calibration needs.

## Key findings

- 4454 high-quality galaxy redshifts obtained so far.
- Galaxy brightness is weakly correlated with redshift in color space.
- Survey data supports calibration for Euclid and WFIRST requirements.

## Abstract

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.