AEGIS: The color-magnitude relation for X-ray-selected active galactic nuclei

K. Nandra, A. Georgakakis, C. N.A. Willmer, M. C. Cooper, D. J. Croton, M. Davis, S. M. Faber, D. C. Koo, E. S. Laird, J. A. Newman

Research output: Contribution to journalArticlepeer-review

200 Scopus citations


We discuss the relationship between rest-frame color and optical luminosity for X-ray sources in the range 0.6 < z < 1.4 selected from the Chandra survey of the Extended Groth Strip. These objects are almost exclusively active galactic nuclei (AGNs). While there are a few luminous QSOs, most are relatively weak or obscured AGNs whose optical colors should be dominated by host galaxy light. The vast majority of AGN hosts at z ∼ 1 are luminous and red, with very few objects fainter than MB = -20.5 or bluer than U - B = 0.6. This places the AGNs in a distinct region of color-magnitude space, on the "red sequence" or at the top of the "blue cloud," with many in between these two modes in galaxy color. A key stage in the evolution of massive galaxies is when star formation is quenched, resulting in a migration from the blue cloud to the red sequence. Our results are consistent with scenarios in which AGNs either cause or maintain this quenching. The large number of red-sequence AGNs implies that strong, ongoing star formation is not a necessary ingredient for AGN activity, as black hole accretion appears often to persist after star formation has been terminated.

Original languageEnglish (US)
Pages (from-to)L11-L14
JournalAstrophysical Journal
Issue number1 II
StatePublished - May 1 2007


  • Galaxies: active
  • Galaxies: evolution
  • Galaxies: nuclei
  • X-rays: galaxies

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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