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. 
Mauricio Cisternas, Knud Jahnke, Angela Bongiorno, Katherine J.   Inskip, Chris D. Impey, Anton M. Koekemoer, Andrea Merloni...
astro-ph.CO astro-ph.GA