Galaxies in COSMOS: Evolution of Black hole vs. bulge mass but not vs. total stellar mass over the last 9 Gyrs?
by Jahnke, Knud and Bongiorno, Angela and Brusa, Marcella and Capak, Peter and Cappelluti, Nico and Cisternas, Mauricio and Civano, Francesca and Colbert, James and Comastri, Andrea and Elvis, Martin and Hasinger, Günther and Impey, Chris and Inskip, Katherine and Koekemoer, Anton M. and Lilly, Simon and Maier, Christian and Merloni, Andrea and Riechers, Dominik and Salvato, Mara and Schinnerer, Eva and Scoville, Nick Z. and Silverman, John and Taniguchi, Yoshi and Trump, Jonathan R. and Yan, Lin
Submitted to ApJL; 7 pages, 2 figures
We constrain the ratio of black hole (BH) mass to total stellar mass of type-1 AGN in the COSMOS survey at 1<z<2. For 10 AGN at mean redshift z~1.4 with both HST/ACS and HST/NICMOS imaging data we are able to compute total stellar mass M_(*,total), based on restframe UV-to-optical host galaxy colors which constrain mass-to-light ratios. All objects have virial BH mass-estimates available from the COSMOS Magellan/IMACS and zCOSMOS surveys. We find 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: (a) If our objects were purely bulge-dominated, the M_BH–M_(*,bulge)-relation has not evolved since z~1.4. However, (b) since we have evidence for substantial disk components, the bulges of massive galaxies (logM_(*,total)=11.1+-0.25 or logM_BH~8.3+-0.2) must have grown over the last 9 Gyrs predominantly by redistribution of disk- into bulge-mass. Since all necessary stellar mass exists in the galaxy at z=1.4, no star-formation or addition of external stellar material is required, 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.
