Growing Massive Black Holes in a Local Group Environment: the Central Supermassive, Slowly Sinking, and Ejected Populations

TL;DR

This study models the growth of small black holes in dwarf galaxies within a Local Group environment, predicting populations of massive, ejected, and wandering black holes based on merger-driven growth scenarios.

Contribution

It introduces a new model allowing for low-level gas accretion during mergers and predicts black hole populations in dwarf galaxies and the intergalactic medium.

Findings

Up to 35 black holes < 10^6 Msun in Local Group dwarf galaxies.

Hundreds of black holes < 10^5 Msun may be ejected into intergalactic space.

Predicted black hole populations can help constrain Sgr A* growth mechanisms.

Abstract

We explore the growth of < 10^7 Msun black holes that reside at the centers of spiral and field dwarf galaxies in a Local Group type of environment. We use merger trees from a cosmological N-body simulation known as Via Lactea II (VL-2) as a framework to test two merger-driven semi-analytic recipes for black hole growth that include dynamical friction, tidal stripping, and gravitational wave recoil in over 20,000 merger tree realizations. First, we apply a Fundamental Plane limited (FPL) model to the growth of Sgr A*, which drives the central black hole to a maximum mass limited by the Black Hole Fundamental Plane after every merger. Next, we present a new model that allows for low-level Prolonged Gas Accretion (PGA) during the merger. We find that both models can generate a Sgr A* mass black hole. We predict a population of massive black holes in local field dwarf galaxies - if the VL-2 simulation is representative of the growth of the Local Group, we predict up to 35 massive black holes (< 10^6 Msun) in Local Group field dwarfs. We also predict that hundreds of < 10^5 Msun black holes fail to merge, and instead populate the Milky Way halo, with the most massive of them at roughly the virial radius. In addition, we find that there may be hundreds of massive black holes ejected from their hosts into the nearby intergalactic medium due to gravitational wave recoil. We discuss how the black hole population in the Local Group field dwarfs may help to constrain the growth mechanism for Sgr A*.