HAWK-I imaging of the X-ray luminous galaxy cluster XMMU J2235.3-2557: The red sequence at z=1.39

TL;DR

This study uses near-infrared imaging to analyze the galaxy population of a distant, X-ray luminous galaxy cluster at z=1.39, revealing a well-established red sequence in the core and ongoing galaxy evolution in the outskirts.

Contribution

First high-resolution near-IR imaging of XMMU J2235.3-2557 at z=1.39, showing the formation and evolution of the red sequence in a distant galaxy cluster.

Findings

Core galaxies are already 3 Gyr old at z=1.39.

Red sequence is well-defined in the core, with increased star formation in outskirts.

Red sequence is forming from the inside out.

Abstract

We use HAWK-I, the recently-commissioned near-IR imager on Yepun (VLT-UT4), to obtain wide-field, high-resolution images of the X-ray luminous galaxy cluster XMMU J2235.3-2557 in the J and Ks bands, and we use these images to build a colour-magnitude diagram of cluster galaxies. Galaxies in the core of the cluster form a tight red sequence with a mean J-Ks colour of 1.9 (Vega system). The intrinsic scatter in the colour of galaxies that lie on the red sequence is similar to that measured for galaxies on the red sequence of the Coma cluster. The slope and location of the red sequence can be modelled by passively evolving the red sequence of the Coma cluster backwards in time. Using simple stellar population (SSP) models, we find that galaxies in the core of XMMU J2235.3-2557 are, even at z=1.39, already 3 Gyr old, corresponding to a formation redshift of z ~ 4. Outside the core, the intrinsic scatter and the fraction of galaxies actively forming stars increase substantially. Using SSP models, we find that most of these galaxies will join the red sequence within 1.5 Gyr. The contrast between galaxies in the cluster core and galaxies in the cluster outskirts indicates that the red sequence of XMMU J2235.3-2557 is being built from the dense cluster core outwards.