Detecting floating black holes as they traverse the gas disc of the Milky Way

A population of intermediate-mass black holes (BHs) is predicted to be freely floating in the Milky Way (MW) halo, due to gravitational wave recoil, ejection from triple BH systems, or tidal stripping in the dwarf galaxies that merged to make the MW. As these BHs traverse the gaseous MW disc, a bow shock forms, producing detectable radio synchrotron emission from accelerated electrons. We calculate the synchrotron flux to be similar to 0.01-10 mJy at GHz frequency, detectable by Jansky Very Large Array, and similar to 0.1-1 mu Jy in the infrared, detectable by Hubble Space Telescope and James Webb Space Telescope. The discovery of the floating BH population will provide insights on the formation and merger history of the MW as well as on the evolution of massive BHs in the early Universe.