Hunting for wandering massive black holes
  We investigate low-density accretion flows onto massive black holes (BHs)
with masses of $\gtrsim 10^5~M_\odot$ orbiting around in the outskirts of their
host galaxies, performing three-dimensional hydrodynamical simulations. Those
wandering BHs are populated via ejection from the galactic nuclei through
multi-body BH interactions and gravitational wave recoils associated with
galaxy and BH coalescences. We find that when a wandering BH is fed with hot
and diffuse plasma with density fluctuations, the mass accretion rate is
limited at $\sim 10-20\%$ of the canonical Bondi-Hoyle-Littleton rate owing to
a wide distribution of inflowing angular momentum. We further calculate
radiation spectra from radiatively inefficient accretion flows onto the
wandering BH using a semi-analytical two-temperature disk model and find that
the predicted spectra have a peak at the millimeter band, where the Atacama
Large Millimeter/submillimeter Array (ALMA) has the highest sensitivity and
spatial resolution. Millimeter observations with ALMA and future facilities
such as the next generation Very Large Array (ngVLA) will enable us to hunt for
a population of wandering BHs and push the detectable mass limit down to
$M_\bullet \simeq 2\times10^7~M_\odot$ for massive nearby ellipticals, e.g.,
M87, and $M_\bullet \simeq 10^5~M_\odot$ for the Milky Way. This radiation
spectral model, combined with numerical simulations, will be applied to give
physical interpretations of off-nuclear BHs detected in dwarf galaxies, which
may constrain BH seed formation scenarios.
