Direct collapse black hole formation and assembly in cosmological simulations

Abstract

Observations of our own galaxy and others near and far demonstrate that massive black holes live at the centers of most galaxies. The origin of these black holes is the subject of much debate. We study the birth of these supermassive black holes from the direct collapse process and characterize sites where these black hole seeds form. This work tests the power of the direct collapse model in a self-consistent, time-dependent, non-uniform Lyman-Werner radiation field using an updated version of the N-body+SPH code Gasoline. Our results from this work demonstrate that, in contrast with existing models that restrict direct collapse to completely pristine halos, direct collapse black hole seeds can form in rare pockets of low-metallicity gas in halos with recent star formation. We then combine semi-analytic modeling with these cosmological zoom-in simulations of a Milky Way-type galaxy to investigate the role of black hole spin and gravitational recoil in the epoch of massive black hole seeding. When two black holes merge, the asymmetric emission of gravitational waves provides an impulse to the merged system. This gravitational wave recoil velocity can be up to 4000 km/s, easily large enough for the black hole to escape its host galaxy. We sample four different spin alignment distributions and compare the resulting merger histories, occupation fractions, and black hole-host relations with what is expected by excluding the effect of recoil. The inclusion of gravitational recoil and black hole spin in the assembly of massive black hole seeds can reduce the final z=5 black hole mass by more than an order of magnitude. The black hole occupation fraction, however, remains effectively unaltered due to repeated episodes of black hole formation following a recoil event. While electromagnetic detections of these events are unlikely, the upcoming Laser Interferometer Space Antenna (LISA) is ideally suited to detect gravitational wave signals from such events.