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

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