A hidden population of massive black holes in simulated dwarf galaxies
  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 $z=2$.
