Active Galactic Nuclei and the Truncation of Star Formation in K+A Galaxies

We have searched for active galactic nuclei (AGNs) in K+A galaxies, using multiwavelength imaging and spectroscopy in the Bootes 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 H-alpha 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 M_R<-22 K+A galaxies host AGNs with X-ray luminosities of 10^{42} erg/s. 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.

💡 Research Summary

This study investigates the prevalence of active galactic nuclei (AGN) in post‑starburst, or “K+A”, galaxies using multi‑wavelength imaging and spectroscopy in the Boötes field of the NOAO Deep Wide‑Field Survey. K+A galaxies are identified by strong Balmer absorption lines (indicative of a recent burst of star formation) combined with weak Hα emission, signifying that star formation has been abruptly truncated. From a parent sample of 6,594 galaxies with 0.10 < z < 0.35 and I < 20, the authors isolate 24 K+A systems based on the criteria set by the AGN and Galaxy Evolution Survey (AGES).

A visual inspection of deep optical images reveals that roughly two‑thirds of the K+A galaxies show clear signs of ongoing or recent mergers: close companions, tidal tails, or disturbed morphologies. This high merger fraction suggests that either the merger itself directly quenches star formation, or that we are observing the aftermath of a merger‑driven starburst that has now ceased.

In colour–magnitude space the K+A objects occupy the “green valley” between the blue cloud of star‑forming galaxies and the red sequence of passive systems. However, only about 1 % of all galaxies with intermediate colours are classified as K+A, implying that the transition through this phase must be extremely rapid—on the order of ≤100 Myr. Such a short timescale disfavors gradual gas depletion and instead points to a violent quenching mechanism.

To assess AGN activity, the authors employ two complementary diagnostics. First, they plot the galaxies on Baldwin‑Phillips‑Terlevich (BPT) diagrams using the emission‑line ratios