Do Moderate-Luminosity Active Galactic Nuclei Suppress Star Formation?
  The growth of supermassive black holes and their host galaxies are thought to
be linked, but the precise nature of this symbiotic relationship is still
poorly understood. Both observations and simulations of galaxy formation
suggest that the energy input from active galactic nuclei (AGN), as the central
supermassive black hole accretes material and grows, heats the interstellar
material and suppresses star formation. In this Letter, we show that most host
galaxies of moderate-luminosity supermassive black holes in the local universe
have intermediate optical colors that imply the host galaxies are transitioning
from star formation to quiescence, the first time this has been shown to be
true for all AGN independent of obscuration. The intermediate colors suggest
that star formation in the host galaxies ceased roughly 100 Myr ago. This
result indicates that either the AGN are very long-lived, accreting for more
than 1 Gyr beyond the end of star formation, or there is a ~100 Myr time delay
between the shutdown of star formation and detectable black hole growth. The
first explanation is unlikely given current estimates for AGN lifetimes, so
low-lumiosity AGN must shut down star formation before substantial black hole
accretion activity is detected. The scarcity of AGN host galaxies in the blue
cloud reported here challenges scenarios where significant star formation and
black hole growth are coeval. Lastly, these observations also strongly support
the `Unified Model' of AGN as the host galaxy colors are independent of
obscuration towards the central engine.
