Massive galaxies in cosmos: Evolution of black hole versus bulge mass but not versus total stellar mass over the last 9 Gyr?

We constrain the ratio of black hole (BH) mass to total stellar mass of type-1 active galactic nuclei (AGNs) in the COSMOS survey at 1 < z < 2. For 10 AGNs at mean redshift z ∼ 1.4 with both Hubble Space Telescope (HST)/ACS and HST/NICMOS imaging data, we are able to compute the total stellar mass M _*,total, based on rest-frame UV-to-optical host galaxy colors which constrain mass-to-light ratios. All objects have virial M _BH estimates available from the COSMOS Magellan/IMACS and zCOSMOS surveys. We find within errors zero difference between the M _BH-M _*,total relation at z ∼ 1.4 and the M _BH-M _*,bulge relation in the local universe. Our interpretation is (1) if our objects were purely bulge-dominated, the M _BH-M _*,bulge relation has not evolved since z ∼ 1.4. However, (2) since we have evidence for substantial disk components, the bulges of massive galaxies (M _*,total = 11.1 ± 0.3 or log M _BH ∼ 8.3 ± 0.2) must have grown over the last 9 Gyr predominantly by redistribution of the disk into the bulge mass. Since all necessary stellar mass exists in galaxies at z = 1.4, no star formation or addition of external stellar material is required, but only a redistribution, e.g., induced by minor and major merging or through disk instabilities. Merging, in addition to redistributing mass in the galaxy, will add both BH and stellar/bulge mass, but does not change the overall final M _BH/M _*,bulge ratio. Since the overall cosmic stellar and BH mass buildup trace each other tightly over time, our scenario of bulge formation in massive galaxies is independent of any strong BH feedback and means that the mechanism coupling BH and bulge mass until the present is very indirect.