Galaxy Clusters in Formation: Determining the Age of the Red-Sequence in Optical and X-ray Clusters at z~1 with HST

TL;DR

This study compares the color-magnitude relations of galaxy clusters at z~1 in optical and X-ray samples, revealing differences in galaxy evolution and star-formation truncation epochs using HST imaging.

Contribution

It provides the first direct comparison of optical and X-ray selected clusters at z~1, linking galaxy evolution stages to cluster selection methods.

Findings

Optical clusters have a larger CMR scatter than X-ray clusters.

Star formation in optical clusters truncated at z~1.6, later than in X-ray clusters at z~2.1.

Optical clusters contain fewer E/S0 galaxies in their cores.

Abstract

Using deep two-band imaging from the Hubble Space Telescope, we measure the color-magnitude relations (CMR) of E/S0 galaxies in a set of 9 optically-selected clusters principally from the Red-Sequence Cluster Survey (RCS) at 0.9 < z < 1.23. We find that the mean scatter in the CMR in the observed frame of this set of clusters is 0.049 +/- 0.008, as compared to 0.031 +/- 0.007 in a similarly imaged and identically analyzed X-ray sample at similar redshifts. Single-burst stellar population models of the CMR scatter suggest that the E/S0 population in these RCS clusters truncated their star-formation at z~1.6, some 0.9 Gyrs later than their X-ray E/S0 counterparts which were truncated at z~2.1. The notion that this is a manifestation of the differing evolutionary states of the two populations of cluster galaxies is supported by comparison of the fraction of bulge-dominated galaxies found in the two samples which shows that optically-selected clusters contain a smaller fraction of E/S0 galaxies at the their cores