Observational evidence for AGN feedback in early-type galaxies

TL;DR

This study provides observational evidence that AGN feedback plays a role in quenching star formation in early-type galaxies, showing an evolutionary sequence from star formation to quiescence linked to nuclear activity.

Contribution

It offers the first large-scale observational evidence supporting AGN feedback's role in galaxy evolution, using a novel morphological selection and multiwavelength analysis.

Findings

AGN host galaxies are closer to the red sequence.

Star formation occurred several hundred Myr ago in these galaxies.

The transition from star formation to quiescence lasts about 1 Gyr.

Abstract

A major amendment in recent models of hierarchical galaxy formation is the inclusion of so-called 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<z<0.1 from the SDSS database. Key in our approach is the use of a purely morphological selection criterion through visual inspection which produces a sample that is not biased against recent star formation and nuclear activity. 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-UV to near-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.