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 > 10(10) L-circle dot) 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 > 10(42) 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.