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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.