AGN in dusty hosts: implications for galaxy evolution

TL;DR

This study provides empirical evidence linking starburst activity and luminous AGN phases in dusty galaxy hosts, suggesting AGN may influence the cessation of star formation in galaxy evolution.

Contribution

It demonstrates a strong association between IR-luminous star-forming galaxies and type-2 AGN hosts, highlighting a coeval phase and transitional states in galaxy evolution.

Findings

IR-luminous galaxies are 3 times more likely to host type-2 AGN.

AGN hosts often occupy the same colour space as dusty star-forming galaxies.

Some AGN hosts are offset, indicating transitional or post-starburst phases.

Abstract

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.