No evidence for excess AGN activity in recently quenched massive galaxies at cosmic noon

Abstract

We present an analysis of AGN activity within recently quenched massive galaxies at cosmic noon ( z ∼2), using deep Chandra X-ray observations of the Ultra-Deep Surv e y (UDS) field. Our sample includes o v er 4000 massive galaxies ( M*> 10 10 . 5 M ⊙) in the redshift range 1 < z < 3, including more than 200 transitionary post-starburst (PSB) systems. We find that X-ray emitting AGN are detected in 6 . 2 ±1 . 5 per cent of massive PSBs at these redshifts, a detection rate that lies between those of star-forming and passive galaxies (8 . 2 ±0 . 5 per cent and 5 . 7 ±0 . 8 per cent, respectively). A stacking analysis shows that the average X-ray luminosity for PSBs is comparable to older passive galaxies, but a factor of 2 . 6 ±0 . 3 below star-forming galaxies of similar redshift and stellar mass. The average X-ray luminosity in all populations appears to trace the star formation rate, with PSBs sho wing lo w le vels of AGN acti vity consistent with their reduced levels of star formation. We conclude that, on average, we see no evidence for excess AGN activity in the PSB phase. Ho we ver, the lo w le vels of AGN acti vity can be reconciled with the high-v elocity outflows observ ed in man y PSBs, assuming the rare X-ray detections represent short-lived bursts of black hole activity, visible ∼5 per cent of the time. Thus, X-ray AGN may help to maintain quiescence in massive galaxies at cosmic noon, but the evidence for a direct link to the primary quenching event remains elusive.