Early-type galaxies at z = 1.3. I. The Lynx supercluster: cluster and groups at z=1.3. Morphology and color-magnitude relation

Abstract

We confirm the detection of 3 groups in the Lynx supercluster, at z~1.3, and give their redshifts and masses. We study the properties of the group galaxies as compared to the central clusters, RXJ0849+4452 and RXJ0848+4453, selecting 89 galaxies in the clusters and 74 galaxies in the groups. We morphologically classify galaxies by visual inspection, noting that our early-type galaxy (ETG) sample would have been contaminated at the 30% -40% level by simple automated classification methods (e.g. based on Sersic index). In luminosity selected samples, both clusters and groups show high fractions of Sa galaxies. The ETG fractions never rise above ~50% in the clusters, which is low compared to the fractions observed in clusters at z~1. However, ETG plus Sa fractions are similar to those observed for ETGs in clusters at z~1. Bulge-dominated galaxies visually classified as Sas might also be ETGs with tidal features or merger remnants. They are mainly red and passive, and span a large range in luminosity. Their star formation seems to have been quenched before experiencing a morphological transformation. Because their fraction is smaller at lower redshifts, they might be the spiral population that evolves into ETGs. For mass-selected samples, the ETG fraction show no significant evolution with respect to local clusters, suggesting that morphological transformations occur at lower masses and densities. The ETG mass-size relation shows evolution towards smaller sizes at higher redshift in both clusters and groups, while the late-type mass-size relation matches that observed locally. The group ETG red sequence shows lower zero points and larger scatters than in clusters, both expected to be an indication of a younger galaxy population. The estimated age difference is small when compared to the difference in age at different galaxy masses.