Galaxies and massive black holes (BHs) are presumed to grow via galactic merging events
and subsequent BH coalescence.

We investigate the merging event between the Andromeda galaxy (M31) and a satellite
galaxy (Miki et al. 2014).  We compute the expected observational appearance of the
massive BH and stars around it that were at the center of the satellite galaxy prior to
the merger, and are currently wandering in the M31 halo (Kawaguchi et al. 2014). The
expected broadband spectrum of an accretion flow onto the BH indicates that the radio
band is the best frequency range to detect the emission.

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 imaging of BH
shadows that avoids strong interstellar scattering in the center of galaxies.

We will also discuss recent radio/optical/X-ray observations on nearby 3 candidates of
wandering massive black holes and trailing stars.