  Galaxies and massive black holes (BHs) presumably grow via galactic merging
events and subsequent BH coalescence. As a case study, we investigate the
merging event between the Andromeda galaxy (M31) and a satellite galaxy. We
compute the expected observational appearance of the massive BH that was at the
center of the satellite galaxy prior to the merger, and is currently wandering
in the M31 halo. We demonstrate that a radiatively inefficient accretion flow
with a bolometric luminosity of a few tens of solar luminosities develops when
Hoyle-Lyttleton accretion onto the BH is assumed. We compute the associated
broadband spectrum and show that the radio band (observable with EVLA, ALMA and
SKA) is the best frequency range to detect the emission. We also evaluate the
mass and the luminosity of the stars bound by the wandering BH and find that
such a star cluster is sufficiently luminous that it could correspond to one of
the star clusters found by the PAndAS survey. The discovery of a relic massive
BH wandering in a galactic halo will provide a direct means to investigate in
detail the coevolution of galaxies and BHs. It also means a new population of
BHs (off-center massive BHs), and offers targets for clean BH imaging that
avoids strong interstellar scattering in the center of galaxies.
