Observational evidence for AGN feedback in early-type galaxies

A major amendment in recent models of hierarchical galaxy formation is the inclusion of so-called active galactic nucleus (AGN) feedback. The energy input from an active central massive black hole is invoked to suppress star formation in early-type galaxies at later epochs. A major problem is that this process is poorly understood, and compelling observational evidence for its mere existence is still missing. In search for signatures of AGN feedback, we have compiled a sample of 16 000 early-type galaxies in the redshift range 0.05 200 km s−1) and roughly evenly distributed between star formation and AGN at intermediate and low (σ < 100 km s−1) masses. The objects with emission (∼20 per cent) are offset from the red sequence and form a well-defined pattern in the colour–mass diagram. Star-forming early-types inhabit the blue cloud, while early-types with AGN are located considerably closer to and almost on the red sequence. Star formation–AGN composites are found right between these two extremes. We further derive galaxy star formation histories using a novel method that combines multiwavelength photometry from near-ultraviolet (UV) to near-infrared (IR) and stellar absorption indices. We find that in those objects deviating from the red sequence star formation occurred several 100 Myr in the past involving 1–10 per cent of the total stellar mass. We identify an evolutionary sequence from star formation via nuclear activity to quiescence. This transition process lasts about 1 Gyr, and the peak AGN phase occurs roughly half a Gyr after the starburst. The most likely interpretation is that star formation is suppressed by nuclear activity in these objects before they settle on the red sequence. This is empirical evidence for the occurrence of AGN feedback in early-type galaxies at recent epochs.