AGN in dusty hosts: implications for galaxy evolution

We present strong empirical evidence for a physical connection between the occurrence of a starburst (SB) and a luminous active galactic nucleus (AGN) phase. Drawing infrared (IR), X-ray and optically selected samples from COSMOS, we find that the locus of type II AGN hosts in the optical colour-magnitude (U - V/V) and colour-colour (U - V/V - J) space significantly overlaps with that of IR-luminous (L_IR > 10¹⁰ L_⊙) galaxies. Based on our observations, we propose that, when simultaneously building their black hole and stellar masses, type II AGN hosts are located in the same part of colour-colour space as dusty star-forming galaxies. In fact, our results show that IR-luminous galaxies at z < 1.5 are on average three times more likely to host a type II AGN (L_X > 10⁴² erg s^-1) than would be expected serendipitously, if AGN and star-formation events were unrelated. In addition, the optical and IR properties of the AGN/SB hybrid systems tentatively suggest that the AGN phase might be coeval with a particularly active phase in a galaxy's star-formation history. Interestingly, we also find a significant fraction of type II AGN hosts offset from the dusty galaxy sequence in colour-colour space, possibly representing a transitional or post-SB phase in galaxy evolution. Our findings are consistent with a scenario whereby AGN play a role in the termination of star formation in massive galaxies.