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 AGN phase. Drawing infrared (IR),
X-ray, and optically selected samples from COSMOS, we find that the locus of
type-2 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_sun) galaxies. Based on our observations, we propose that, when
simultaneously building their black hole and stellar masses, type-2 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 3 times more likely to host a type-2 AGN (L_X > 10^42 erg/s) than would
be expected serendipitously, if AGN and star-formation events were unrelated.
In addition, the optical and infrared 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-2 AGN hosts offset from the dusty galaxy
sequence in colour-colour space, possibly representing a transitional or
post-starburst 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.
