Abstract:
When star formation stops in blue star-forming galaxies, their stars evolve
passively through intermediate colors ("the green valley") and then onto the
red sequence. The high incidence of AGN in the green valley has been
interpreted as evidence that AGN feedback is what suppresses star formation.
Using new observations from the IRAM 30m and Swift BAT, I show that the role
of AGN in the shutdown of star formation is much more complex than previously
thought. Specifically, I show that the luminous AGN whose host galaxies lie in
the green valley turn on at least 100 Myr after star formation turns off, and
thus cannot be responsible for its suppression. On the other hand, the
molecular gas that fuels star formation is destroyed extremely rapidly,
possibly by injection of radiation or kinetic energy during the low-luminosity
precursor AGN phase. Simulations do suggest the possibility of an early, weaker
AGN phase, during which kinetic outflows might provide the most important
feedback, followed roughly 100 Myr later by a more luminous AGN phase, as
increasing amounts of accreting material lose enough angular momentum to fall
into the black hole.