Black hole fuelling in galaxy mergers: a high-resolution analysis

Abstract

ABSTRACT Using parsec scale resolution hydrodynamical adaptive mesh refinement simulations, we have studied the mass transport process throughout a Galactic merger. The aim of such study is to connect both the peaks of mass accretion rate on to the BHs and star formation bursts with both gravitational and hydrodynamic torques acting on the galactic gaseous component. Our merger initial conditions were chosen to mimic a realistic system. The simulations include gas cooling, star formation, supernovae feedback, and AGN feedback. Gravitational and hydrodynamic torques near pericentre passes trigger gas funneling to the nuclei that is associated with bursts of star formation and black hole growth. Such episodes are intimately related with both kinds of torques acting on the galactic gas. Pericentres trigger both star formation and mass accretion rates of ∼few $(1\!-\!10)\, \mathrm{ M}_\odot$ yr−1. Such episodes last $\sim \, (50\!-\!75)$ Myr. Close passes also can produce black hole accretion that approaches and reaches the Eddington rate, lasting ∼few Myrs. Our simulation shows that both gravitational and hydrodynamic torques are enhanced at pericentre passes with gravitational torques tending to have higher values than the hydrodynamic torques throughout the merger. We also find that in the closest encounters, hydrodynamic and gravitational torques can be comparable in their effect on the gas, the two helping in the redistribution of both angular momentum and mass in the galactic disc. Such phenomena allow inward mass transport on to the BH influence radius, fuelling the compact object and lighting up the galactic nuclei.