Galaxy evolution in overdense environments at high redshift: passive early-type galaxies in a cluster at redshift 2

TL;DR

This study investigates galaxy populations in a high-redshift galaxy cluster at z=2, revealing a mix of passive and active galaxies, and providing insights into galaxy evolution in dense environments 10 billion years ago.

Contribution

It provides the first detailed analysis of galaxy morphology and stellar populations in a z=2 cluster, highlighting accelerated structural evolution in dense environments.

Findings

Passive fraction is higher in the cluster core.

Passive early-type galaxies are smaller than local counterparts.

Cluster hosts both passive and actively star-forming massive galaxies.

Abstract

We present a study of galaxy populations in the central region of the IRAC-selected, X-ray detected galaxy cluster Cl J1449+0856 at z=2. Based on a sample of spectroscopic and photometric cluster members, we investigate stellar populations and morphological structure of cluster galaxies over an area of ~0.7Mpc^2 around the cluster core. The cluster stands out as a clear overdensity both in redshift space, and in the spatial distribution of galaxies close to the center of the extended X-ray emission. The cluster core region (r<200 kpc) shows a clearly enhanced passive fraction with respect to field levels. However, together with a population of massive passive galaxies mostly with early-type morphologies, it also hosts massive actively star-forming, often highly dust-reddened sources. Close to the cluster center, a multi-component system of passive and star-forming galaxies could be the future BCG still assembling. We observe a clear correlation between passive stellar populations and an early-type morphology, in agreement with field studies at similar redshift. Passive early-type galaxies in this clusters are typically a factor 2-3 smaller than similarly massive early-types at z~0, but also on average larger by a factor ~2 than their field analogs at z~2, lending support to recent claims of an accelerated structural evolution in high-redshift dense environments. These results point towards the early formation of a population of massive galaxies, already evolved both in their structure and stellar populations, coexisting with still-actively forming massive galaxies in the central regions of young clusters 10 billion years ago.