THE EVOLUTION OF BLACK HOLE SCALING RELATIONS IN GALAXY MERGERS

We study the evolution of black holes (BHs) on the M-BH-sigma and M-BH-M-bulge planes as a function of time in disk galaxies undergoing mergers. We begin the simulations with the progenitor BH masses being initially below (Delta logM(BH,i) similar to -2), on (Delta logM(BH,i) similar to 0), and above (Delta logM(BH,i) similar to 0.5) the observed local relations. The final relations are rapidly established after the final coalescence of the galaxies and their BHs. Progenitors with low initial gas fractions (f(gas) = 0.2) starting below the relations evolve onto the relations (Delta logM(BH,f) similar to -0.18), progenitors on the relations stay there (Delta logM(BH,f) similar to 0), and finally progenitors above the relations evolve toward the relations, but still remain above them (Delta logM(BH,f) similar to 0.35). Mergers in which the progenitors have high initial gas fractions (f(gas) = 0.8) evolve above the relations in all cases (Delta logM(BH,f) similar to 0.5). We find that the initial gas fraction is the prime source of scatter in the observed relations, dominating over the scatter arising from the evolutionary stage of the merger remnants. The fact that BHs starting above the relations do not evolve onto the relations indicates that our simulations rule out the scenario in which overmassive BHs evolve onto the relations through gas-rich mergers. By implication our simulations thus disfavor the picture in which supermassive BHs develop significantly before their parent bulges.