Active galactic nuclei and the truncation of star formation in K+A galaxies

Michael J.I. Brown, John Moustakas, Nelson Caldwell, David Palamara, Richard J. Cool, Arjun Dey, Ryan C. Hickox, Buell T. Jannuzi, Stephen S. Murray, Dennis Zaritsky

Research output: Contribution to journalArticlepeer-review

19 Scopus citations


We have searched for active galactic nuclei (AGNs) in K+A galaxies, using multiwavelength imaging and spectroscopy in the Boötes field of the NOAO Deep Wide-Field Survey. The K+A galaxies, which have had their star formation rapidly truncated, are selected via their strong Balmer absorption lines and weak Hα emission. Our sample consists of 24 K+A galaxies selected from 6594 0.10 < z < 0.35 galaxies brighter than I = 20 with optical spectroscopy from the AGN and Galaxy Evolution Survey. Two thirds of the K+A galaxies are likely ongoing galaxy mergers, with nearby companion galaxies or tidal tails. Galaxy mergers may be responsible for the truncation of star formation, or we are observing the aftermath of merger triggered starbursts. As expected, the optical colors of K+A galaxies largely fall between blue galaxies with ongoing star formation and red passive galaxies. However, only 1% of the galaxies with colors between the red and blue populations are K+A galaxies, and we conclude that the truncation of star formation in K+A galaxies must have been unusually abrupt (≲100 Myr). We examined the AGN content of K+A galaxies with both optical emission-line ratios (BPT diagrams) and Chandra X-ray imaging. At least half of all K+A galaxies display the optical emission-line ratios of AGNs, and a third of MR < -22 K+A galaxies host AGNs with X-ray luminosities of 1042 erg s-1. The faintest K+A galaxies do not show clear evidence for hosting AGNs, having emission-line ratios consistent with photoionization by massive stars and few X-ray detections. We speculate that two mechanisms may be responsible for the truncation of star formation in K+A galaxies, with AGN feedback only playing a role in M R ≲ -20.5 galaxies.

Original languageEnglish (US)
Pages (from-to)150-158
Number of pages9
JournalAstrophysical Journal
Issue number1
StatePublished - 2009


  • Active
  • Evolution
  • Galaxies
  • Interactions
  • X-rays

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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