Euclid preparation: VIII. The Complete Calibration of the Colour-Redshift Relation (C3R2) Survey: VLT/KMOS Observations and Data Release

The Complete Calibration of the Color-Redshift Relation (C3R2) survey is a spectroscopic effort involving ESO and Keck facilities designed specifically to empirically calibrate the galaxy colour-redshift relation — P(zjC) to the Euclid depth (iAB = 24:5) and is intimately linked to the success of upcoming Stage IV dark energy missions based on weak lensing cosmology. The aim is to build a spectroscopic calibration sample that is as representative as possible of the galaxies of the Euclid weak lensing sample. In order to minimize the number of spectroscopic observations necessary to fill the gaps in the current knowledge of the P(zjC), self-organizing maps (SOM) representations of the galaxy colour space have been constructed. Here we present the first results of an ESO@VLT Large Programme approved in the context of C3R2, that makes use of the two VLT optical and near-infrared multi-object spectrographs, FORS2 and KMOS. This data release paper focuses on high-quality spectroscopic redshifts of high-redshift galaxies observed with the KMOS spectrograph in the near-infrared H- and K-bands. A total number of 424 highly-reliable redshifts are measured in the range 1:3 ≤ z ≤ 2:5, with a total success rate of 60.7% in the H-band and 32.8% in the K-band. The newly determined redshifts fill the 55% of high (mainly regions with no spectroscopic measurements) and 35% of lower (regions with low-resolution/low-quality spectroscopic measurements) priority empty SOM grid cells. We measure Hα fluxes in a 10:02 radius aperture from the spectra of the spectroscopically confirmed galaxies and convert them into star-formation rates. In addition, we performed SED fitting analysis on the same sample in order to derive stellar masses, E(B - V), total magnitudes, and SFRs. We combine the results obtained from the spectra with those derived via SED fitting, and we show that the spectroscopic failures come from either weakly star-forming galaxies (at z < 1:7, i.e. in the H-band) or low S/N spectra (in the K-band) of z > 2 galaxies.