Euclid preparation: XX. The Complete Calibration of the Color-Redshift Relation survey: LBT observations and data release

TL;DR

This paper reports on LBT near-infrared spectroscopic observations for the C3R2 survey, aiming to calibrate galaxy color-redshift relations crucial for Euclid's dark energy measurements, by providing new redshifts for galaxies in key color space regions.

Contribution

It presents new high-reliability redshift measurements for 251 galaxies, filling gaps in the color-redshift calibration sample for Euclid, and demonstrates progress in spectroscopic coverage of galaxy color space.

Findings

Redshifts for 251 galaxies in key redshift ranges.

Population of 49 previously unmeasured SOM cells.

Almost doubled the occupation numbers of 153 SOM cells.

Abstract

The Complete Calibration of the Color-Redshift Relation survey (C3R2) is a spectroscopic programme designed to empirically calibrate the galaxy color-redshift relation to the Euclid depth (I_E=24.5), a key ingredient for the success of Stage IV dark energy projects based on weak lensing cosmology. A spectroscopic calibration sample as representative as possible of the galaxies in the Euclid weak lensing sample is being collected, selecting galaxies from a self-organizing map (SOM) representation of the galaxy color space. Here, we present the results of a near-infrared H- and K-bands spectroscopic campaign carried out using the LUCI instruments at the LBT. For a total of 251 galaxies, we present new highly-reliable redshifts in the 1.3<= z <=1.7 and 2<= z<=2.7 ranges. The newly-determined redshifts populate 49 SOM cells which previously contained no spectroscopic measurements and almost double the occupation numbers of an additional 153 SOM cells. A final optical ground-based observational effort is needed to calibrate the missing cells in particular in the redshift range 1.7<= z<=2.7 that lack spectroscopic calibration. In the end, Euclid itself will deliver telluric-free NIR spectra that can complete the calibration.