Detectability of Wandering Intermediate-Mass Black Holes in the Milky
  Way Galaxy from Radio to X-rays
  Intermediate-mass black holes (IMBHs, $10^{3-6} \, \rm{M_\odot}$), are
typically found at the center of dwarf galaxies and might be wandering, thus
far undetected, in the Milky Way (MW). We use model spectra for
advection-dominated accretion flows to compute the typical fluxes, in a range
of frequencies spanning from radio to X-rays, emitted by a putative population
of $10^5 \, \rm{M_\odot}$ IMBHs wandering in five realistic, volume-weighted,
MW environments. We predict that $\sim 27\%$ of the wandering IMBHs can be
detected in the X-ray with Chandra, $\sim 37\%$ in the near-infrared with the
Roman Space Telescope, $\sim 49\%$ in the sub-mm with CMB-S4 and $\sim 57\%$ in
the radio with ngVLA. We find that the brightest fluxes are emitted by IMBHs
passing through molecular clouds or cold neutral medium, where they are always
detectable. We propose criteria to facilitate the selection of candidates in
multi-wavelength surveys. Specifically, we compute the X-ray to optical ratio
($\alpha_{\rm ox}$) and the optical to sub-mm ratio, as a function of the
accretion rate of the IMBH. We show that at low rates the sub-mm emission of
IMBHs is significantly higher than the optical, UV and X-ray emission. Finally,
we place upper limits on the number $N_\bullet$ of these objects in the MW:
$N_\bullet<2000$ and $N_\bullet<100$, based on our detectability expectations
and current lack of detections in molecular clouds and cold neutral medium,
respectively. These predictions will guide future searches of IMBHs in the MW,
which will be instrumental to understanding their demographics and evolution.
