AGN and star formation across cosmic time

Abstract

ABSTRACT We investigate the balance of power between stars and AGN across cosmic history, based on the comparison between the infrared (IR) galaxy luminosity function (LF) and the IR AGN LF. The former corresponds to emission from dust heated by stars and AGN, whereas the latter includes emission from AGN-heated dust only. We find that at all redshifts (at least up to z ∼ 2.5), the high-luminosity tails of the two LFs converge, indicating that the most IR-luminous galaxies are AGN-powered. Our results shed light to the decades-old conundrum regarding the flatter high-luminosity slope seen in the IR galaxy LF compared to that in the UV and optical. We attribute this difference to the increasing fraction of AGN-dominated galaxies with increasing total IR luminosity (LIR). We partition the LIR−z parameter space into a star formation-dominated and an AGN-dominated region, finding that the most luminous galaxies at all epochs lie in the AGN-dominated region. This sets a potential ‘limit’ to attainable star formation rates, casting doubt on the abundance of ‘extreme starbursts’: if AGN did not exist, LIR > 1013 L⊙ galaxies would be significantly rarer than they currently are in our observable Universe. We also find that AGN affect the average dust temperatures (Tdust) of galaxies and hence the shape of the well-known LIR−Tdust relation. We propose that the reason why local ULIRGs are hotter than their high-redshift counterparts is because of a higher fraction of AGN-dominated galaxies amongst the former group.