Dust-Corrected Colors Reveal Bimodality in AGN Host Galaxy Colors at z~1
  Using new, highly accurate photometric redshifts from the MUSYC medium-band
survey in the Extended Chandra Deep Field South (ECDF-S), we fit synthetic
stellar population models to compare AGN host galaxies to inactive galaxies at
0.8 < z < 1.2. We find that AGN host galaxies are predominantly massive
galaxies on the red sequence and in the green valley of the color-mass diagram.
Because both passive and dusty galaxies can appear red in optical colors, we
use rest-frame near-infrared colors to separate passively evolving stellar
populations from galaxies that are reddened by dust. As with the overall galaxy
population, ~25% of the `red' AGN host galaxies and ~75% of the `green' AGN
host galaxies have colors consistent with young stellar populations reddened by
dust. The dust-corrected rest-frame optical colors are the blue colors of
star-forming galaxies, which implies that these AGN hosts are not passively
aging to the red sequence. At z~1, AGN activity is roughly evenly split between
two modes of black hole growth: the first in passively evolving host galaxies,
which may be heating up the galaxy's gas and preventing future episodes of star
formation, and the second in dust-reddened young galaxies, which may be
ionizing the galaxy's interstellar medium and shutting down star formation.
