Unveiling the Population of Wandering Black Holes via Electromagnetic
  Signatures
  While most galaxies appear to host a central supermassive black hole (SMBH),
they are expected to also contain a substantial population of off-center
"wandering" SMBHs naturally produced by the hierarchical merger-driven process
of galaxy assembly. This population has been recently characterized in an
analysis of the Romulus cosmological simulations, which correct for the
dynamical forces on SMBHs without artificially pinning them to halo centers.
Here we predict an array of electromagnetic signatures for these wanderers. The
predicted wandering population of SMBHs from Romulus broadly reproduces the
observed spatial offsets of a recent sample of hyperluminous X-ray sources. We
predict that the sources with the most extreme offsets are likely to arise from
SMBHs within satellite galaxies. These simulations also predict a significant
population of secondary active galactic nuclei (AGN) with luminosities at least
10\% that of the central AGN. The majority of galaxies at $z=4$ that host a
central AGN with bolometric luminosity $L_\mathrm{bol}>10^{42} \ \mathrm{erg}
\; \mathrm{s}^{-1}$ are predicted to host a companion off-center AGN of
comparable brightness. We demonstrate that stacked X-ray observations of
similar mass galaxies may reveal a halo of collective emission attributable to
these wanderers. Finally, because wanderers dominate the population of SMBHs
with masses of $\lesssim 10^7\,M_{\odot}$ in Romulus, they may dominate tidal
disruption event (TDE) rates at these masses if they retain a stellar component
(e.g. a nuclear star cluster). This could warrant an order of magnitude
correction to current theoretically estimated TDE rates at low SMBH masses.
