The evolution of Black Hole scaling relations in galaxy mergers

TL;DR

This study uses simulations to examine how black hole and galaxy properties evolve during mergers, revealing that initial gas content influences the scatter in observed relations and challenging previous growth scenarios.

Contribution

It demonstrates that initial gas fraction is the main factor affecting black hole scaling relation scatter and rules out the idea that overmassive black holes evolve onto these relations via mergers.

Findings

Low gas fraction mergers move black holes onto the relations.

High gas fraction mergers keep black holes above the relations.

Initial gas content dominates scatter in black hole scaling relations.

Abstract

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 black hole masses being initially below (Delta log M_BH=-2), on (Delta log M_BH=0) and above (Delta log M_BH=0.5) the observed local relations. The final relations are rapidly established after the final coalescense 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 log M_BH=-0.18), progenitors on the relations stay there (Delta log M_BH=0) and finally progenitors above the relations evolve towards the relations, but still remaining above them (Delta log M_BH=0.35). Mergers in which the progenitors have high initial gas fractions (f_gas=0.8) evolve above the relations in all cases (Delta log M_BH=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.