Secular evolution and a non-evolving black hole to galaxy mass ratio in
  the last 7 Gyr
  We present new constraints on the ratio of black hole (BH) mass to total
galaxy stellar mass at 0.3<z<0.9 for a sample of 32 type-1 active galactic
nuclei (AGNs) from the XMM-COSMOS survey covering the range
M_BH/M_sun~10^(7.2--8.7). Virial M_BH estimates based on H_beta are available
from the COSMOS Magellan/IMACS survey. We use high-resolution Hubble Space
Telescope (HST) imaging to decompose the light of each type-1 AGN and host
galaxy, and employ a specially-built mass-to-light ratio to estimate the
stellar masses (M_*). The M_BH-M_* ratio shows a zero offset with respect to
the local relation for galactic bulge masses, and we also find no evolution in
the mass ratio M_BH/M_*=(1+z)^{0.02+-0.34} up to z~0.9. Interestingly, at the
high-M_BH end there is a positive offset from the z=0 relation, which can be
fully explained by a mass function bias with a cosmic scatter of 0.3,
reaffirming that the intrinsic distribution is consistent with zero evolution.
From our results we conclude that since z~0.9 no substantial addition of
stellar mass is required: the decline in star formation rates and merger
activity at z<1 support this scenario. Nevertheless, given that a significant
fraction of these galaxies show a disk component, their bulges are indeed
undermassive. This is a direct indication that for the last 7 Gyr the only
essential mechanism required in order that these galaxies obey the z=0 relation
is a redistribution of stellar mass to the bulge, likely driven by secular
processes, i.e., internal instabilities and minor merging.
