Wandering Supermassive Black Holes in Milky Way Mass Halos

TL;DR

This paper uses cosmological simulations to predict the population and distribution of wandering supermassive black holes within Milky Way-like galaxies, revealing their typical numbers, locations, and long-term presence.

Contribution

It provides the first self-consistent prediction of wandering SMBHs in MW-mass halos, including their numbers, spatial distribution, and orbital evolution post-mergers.

Findings

Average of 5.1 ± 3.3 wandering SMBHs within 10 kpc of galaxy centers.

Average of 12.2 ± 8.4 wandering SMBHs within virial radius.

Wandering SMBHs are more likely to be found outside galactic disks.

Abstract

We present a self-consistent prediction from a large-scale cosmological simulation for the population of `wandering' supermassive black holes (SMBHs) of mass greater than $10^6$ M$_{\odot}$ on long-lived, kpc-scale orbits within Milky Way (MW)-mass galaxies. We extract a sample of MW-mass halos from the Romulus25 cosmological simulation (Tremmel et al. 2017), which is uniquely able to capture the orbital evolution of SMBHs during and following galaxy mergers. We predict that such halos, regardless of recent merger history or morphology, host an average of $5.1 \pm 3.3$ SMBHs, including their central black hole, within 10 kpc from the galactic center and an average of $12.2 \pm 8.4$ SMBHs total within their virial radius, not counting those in satellite halos. Wandering SMBHs exist within their host galaxies for several Gyrs, often accreted by their host halo in the early Universe. We find, with $>4\sigma$ significance, that wandering SMBHs are preferentially found outside of galactic disks.