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

We report the discovery of an X-ray group of galaxies located at a high redshift of $ z$$ =$1.61 in the Chandra Deep Field South. Based on 4 Ms Chandra data, the group is first identified as an extended X-ray source. We have used a wealth of deep multi-wavelength data to identify the optical counterpart—our red sequence finder detects a significant over-density of galaxies at $ z$$ \sim$1.6. The brightest group galaxy is spectroscopically confirmed at $ z$$ =$1.61, based on published spectroscopic redshifts. Using this as a central redshift of the group, we measure an X-ray luminosity of $ L_{0.1{\rm-}2.4\rm keV}$$ =$(1.8 $ \pm$0.6) $ \times$10$ ^{43}$erg s$ ^{-1}$, which then translates into a group mass of (3.2 $ \pm$0.8) $ \times$10$ ^{13}$$ M_{\odot}$. This is the lowest-mass group ever confirmed at $ z$$ >$1.5. Deep optical–nearIR images from CANDELS reveal that the group exhibits a surprisingly prominent red sequence, and most of the galaxies are consistent with a formation redshift of $ z_{\rm f}$$ =$3. A detailed analysis of the spectral energy distributions of the group member candidates confirms that most of them are indeed passive galaxies. Furthermore, their structural parameters measured from near-IR CANDELS images show that they are morphologically early-type. The newly identified group at $ z$$ =$1.61 is dominated by quiescent early-type galaxies, and the group appears to be similar to those in the local Universe. One possible difference is the high fraction of AGN—38$ ^{+23}_{\hskip.4mm-20}$% of the bright group member candidates are AGN, which might indicate a role for AGN in the quenching of star formation. However, a statistical sample of high-$ 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.