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 the 4Msec Chandra data , 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 \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 - 2.4 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 .
The 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 _ { 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 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 } % of the bright group member candidates are AGN , which might indicate a role for AGN in 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 .