Massive Black Hole Dynamics in Unequal Mass Galaxy Mergers.

Abstract

Observed scaling relations suggest an evolutionary link between massive black holes (BHs) and their host galaxies. We investigate the coevolution of BH and host in galaxy mergers, which drive active galactic nuclei (AGN) activity, produce BH mergers, and lead to starbursts and morphological transitions in galaxies. Furthermore, we focus on the most cosmologically relevant galaxy mergers: unequal mass mergers at high redshift (z = 3). Using high resolution N-body SPH simulations, we track star formation, BH accretion, and associated feedback as the BHs move from separations of tens of kiloparsecs to tens of parsecs. We focus on the role of merger triggered gaseous inflows in driving both central starbursts and efficient BH accretion. We find that the efficiency of BH pairing depends sensitively on the strength of central star formation in the secondary galaxy. Strong gas inflows build up a central stellar cusp that is denser than the primary galaxy, leading the secondary's nucleus to disrupt the nucleus of the larger primary galaxy and resulting in a short timescale (10-20 Myr) for the formation of a BH binary. If the secondary instead experiences weak inflows and strong ram pressure from the primary's disk, the secondary's nucleus is disrupted due to tidal shocks and binary formation is delayed. We also consider simultaneous accretion onto both BHs, testing when and for how long accretion is triggered at a number of observability thresholds. We find that strong dual AGN activity occurs in the late stages of the mergers, at small separations of a few kiloparsecs. Most of the BH accretion is not simultaneous, limiting the observable dual AGN fraction. Finally, we consider the evolution of BHs in low mass systems with quiet merger histories, probing the distribution of BHs on the low mass end of the observed scaling relations. We evolve a population of seed BHs in a Milky Way halo and find that the BH population in dwarf galaxies has not grown much since formation, providing an indicator of the original BH seed formation mechanism. We derive the BH occupation fraction and mass distribution for a range of dwarf galaxies.