Growth and activity of black holes in galaxy mergers with varying mass ratios

TL;DR

This study uses high-resolution hydrodynamical simulations to analyze how galaxy merger parameters influence supermassive black hole growth, accretion activity, and mass evolution across different merger stages.

Contribution

It provides new insights into the role of initial mass ratio and merger stage in black hole accretion and mass evolution, using detailed simulations.

Findings

Initial mass ratio is the key factor in BH accretion enhancement.

BH mass contrast decreases in minor mergers and increases in major mergers.

Gas losing angular momentum, not necessarily low angular momentum gas, fuels BH growth.

Abstract

We study supermassive black holes (BHs) in merging galaxies, using a suite of hydrodynamical simulations with very high spatial (~10 pc) and temporal (~1 Myr) resolution, where we vary the initial mass ratio, the orbital configuration, and the gas fraction. (i) We address the question of when and why, during a merger, increased BH accretion occurs, quantifying gas inflows and BH accretion rates. (ii) We also quantify the relative effectiveness in inducing AGN activity of merger-related versus secular-related causes, by studying different stages of the encounter: the stochastic (or early) stage, the (proper) merger stage, and the remnant (or late) stage. (iii) We assess which galaxy mergers preferentially enhance BH accretion, finding that the initial mass ratio is the most important factor. (iv) We study the evolution of the BH masses, finding that the BH mass contrast tends to decrease in minor mergers and to increase in major mergers. This effect hints at the existence of a preferential range of mass ratios for BHs in the final pairing stages. (v) In both merging and dynamically quiescent galaxies, the gas accreted by the BH is not necessarily the gas with $low$ angular momentum, but the gas that $loses$ angular momentum.