Origins and Demographics of Wandering Black Holes

TL;DR

This study uses cosmological simulations to characterize wandering black holes, revealing their abundance, origins, and properties, and highlighting their significant contribution to black hole populations especially at high redshifts.

Contribution

The paper introduces a novel simulation approach allowing black holes to evolve dynamically and be offset from halo centers, providing new insights into wandering black hole populations.

Findings

Wandering black holes scale linearly with halo mass, with thousands expected in galaxy clusters.

At high redshifts ($z\,\gtrsim\,4$), wandering black holes outnumber and outshine central supermassive black holes.

Most wanderers originate from disrupted satellite galaxies and remain near their seed masses.

Abstract

We characterise the population of wandering black holes, defined as those physically offset from their halo centres, in the Romulus cosmological simulations. Unlike most other currently available cosmological simulations, black holes are seeded based on local gas properties and are permitted to evolve dynamically without being fixed at halo centres. Tracking these black holes allows us to make robust predictions about the offset population. We find that the number of wandering black holes scales roughly linearly with the halo mass, such that we expect thousands of wandering black holes in galaxy cluster halos. Locally, these wanderers account for around 10 per cent of the local black hole mass budget once seed masses are accounted for. Yet for higher redshifts ($z\gtrsim 4$), wandering black holes both outweigh and outshine their central supermassive counterparts. Most wandering black holes, we find, remain close to the seed mass and originate from the centres of previously disrupted satellite galaxies. While most do not retain a resolved stellar counterpart, those that do are situated farther out at larger fractions of the virial radius. Wanderers with higher luminosities are preferentially at lower radius, more massive, and either closer to their host's mid-planes or associated with a stellar overdensity. This analysis shows that our current census of supermassive black holes is incomplete and that a substantial population of off-centre wanderers likely exists.