A hidden population of massive black holes in simulated dwarf galaxies

TL;DR

This study uses cosmological simulations to analyze the presence, activity, and detectability of massive black holes in dwarf galaxies, revealing a large hidden population of low-luminosity MBHs that were more active in the past.

Contribution

First simulation-based analysis of MBH occupation and activity in dwarf galaxies up to redshift 2, highlighting the prevalence of undetectable, low-luminosity black holes.

Findings

MBH occupation fraction drops below unity in low-mass dwarfs.

Dwarf AGN follow observed scaling relations but have higher active fractions.

76% of MBHs in local dwarf galaxies are undetectable with current X-ray surveys.

Abstract

We explore the characteristics of actively accreting MBHs within dwarf galaxies in the \textsc{Romulus25} cosmological hydrodynamic simulation. We examine the MBH occupation fraction, x-ray active fractions, and AGN scaling relations within dwarf galaxies of stellar mass $10^{8} < M_{\rm star} < 10^{10} M_\odot$ out to redshift $z=2$. In the local universe, the MBH occupation fraction is consistent with observed constraints, dropping below unity at $M_{\rm star} < 3\times10^{10} M_{\odot}$, $M_{\rm 200} < 3\times10^{11} M_\odot$. Local dwarf AGN in \textsc{Romulus25} follow observed scaling relations between AGN x-ray luminosity, stellar mass, and star formation rate, though they exhibit slightly higher active fractions and number densities than comparable x-ray observations. Since $z=2$, the MBH occupation fraction has decreased, the population of dwarf AGN has become overall less luminous, and as a result, the overall number density of dwarf AGN has diminished. We predict the existence of a large population of MBHs in the local universe with low x-ray luminosities and high contamination from x-ray binaries and the hot interstellar medium that are undetectable by current x-ray surveys. These hidden MBHs make up $76\%$ of all MBHs in local dwarf galaxies, and include many MBHs that are undermassive relative to their host galaxy's stellar mass. Their detection relies not only on greater instrument sensitivity but on better modeling of x-ray contaminants or multi-wavelength surveys. Our results indicate dwarf AGN were substantially more active in the past despite being low-luminosity today, and indicate future deep x-ray surveys may uncover many hidden MBHs in dwarf galaxies out to at least $z=2$.