An X-ray detected group of quiescent early-type galaxies at z = 1.6 in the Chandra Deep Field South

Masayuki Tanaka, Alexis Finoguenov, Mohammad Mirkazemi, David J. Wilman, John S. Mulchaey, Yoshihiro Ueda, Yongquan Xue, William N. Brandt, Nico Cappelluti

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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 ~ 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 0.1-2.4keV = (1.8±0.6) × 10 43 erg s -1 , which then translates into a group mass of (3.2±0.8) × 10 13 M⊙. 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 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 -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.

Original languageEnglish (US)
Article number17
JournalPublications of the Astronomical Society of Japan
Issue number1
StatePublished - Feb 25 2013

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science

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    Tanaka, M., Finoguenov, A., Mirkazemi, M., Wilman, D. J., Mulchaey, J. S., Ueda, Y., Xue, Y., Brandt, W. N., & Cappelluti, N. (2013). An X-ray detected group of quiescent early-type galaxies at z = 1.6 in the Chandra Deep Field South. Publications of the Astronomical Society of Japan, 65(1), [17].