AGN feedback works both ways

Simulations of galaxy growth need to invoke strong negative feedback from active galactic nuclei (AGN) to suppress the formation of stars and thus prevent the over-production of very massive systems. While some observations provide evidence for such negative feedback, other studies find either no feedback, or even positive feedback, with increased star formation associated with higher AGN luminosities. Here we report an analysis of several hundred AGN and their host galaxies in the Chandra Deep Field South using X-ray and radio data for sample selection. Combined with archival far infrared data as a reliable tracer of star formation activity in the AGN host galaxies, we find that AGN with pronounced radio jets exhibit a much higher star formation rate than the purely X-ray selected ones, even at the same X-ray luminosities. This difference implies that positive AGN feedback plays an important role, too, and therefore has to be accounted for in all future simulation work. We interpret this to indicate that the enhanced star formation rate of radio selected AGN arises because of jet-induced star formation, as is hinted by the different jet powers among our AGN samples, while the suppressed star formation rate of X-ray selected AGN is caused by heating and photo-dissociation of molecular gas by the hot AGN accretion disc.