An X-ray Detected Group of Quiescent Early-type Galaxies at z=1.6 in the Chandra Deep Field South

TL;DR

This paper reports the discovery of a low-mass, high-redshift galaxy group at z=1.61, characterized by a prominent red sequence of passive, early-type galaxies and a surprisingly high AGN fraction, providing insights into early group evolution.

Contribution

First confirmation of a low-mass galaxy group at z>1.5 with detailed multi-wavelength analysis showing its passive, early-type galaxy population and high AGN fraction.

Findings

Group at z=1.61 has a mass of 3.2×10^{13} M_sun.

Most member galaxies are passive and early-type.

High AGN fraction (~38%) observed in the group.

Abstract

(Abridged) We report the discovery of an X-ray group of galaxies located at a high redshift of z=1.61 in the Chandra Deep Field South. The group is first identified as an extended X-ray source. We use a wealth of deep multi-wavelength data to identify the optical counterpart -- our red sequence finder detects a significant over-density of galaxies at z~1.6 and the brightest group galaxy is spectroscopically confirmed at z=1.61. We measure an X-ray luminosity of L_{0.1-2.4 keV}= 1.8\pm0.6 \times 10^{43} erg/s, which then translates into a group mass of 3.2\pm0.8 \times 10^{13} M_sun. This is the lowest mass group ever confirmed at z>1.5. The deep optical-nearIR images from CANDELS reveal that the group exhibits a surprisingly prominent red sequence. A detailed analysis of the spectral energy distributions of the group member candidates confirms that most of them are indeed passive galaxies. Furthermore, their structural parameters measured from the near-IR CANDELS images show that they are morphologically early-type. The newly identified group at z=1.61 is dominated by quiescent early-type galaxies and the group appears similar to those in the local Universe. One possible difference is the high fraction of AGN (38^{+23}_{-20}%), which might indicate a role for AGN in quenching. But, a statistical sample of high-z groups is needed to draw a general picture of groups at this redshift. Such a sample will hopefully be available in near future surveys.