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