Abstract
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 disk, a bow shock forms, producing detectable radio and mm/sub-mm synchrotron emission from accelerated electrons. We calculate the synchrotron flux to be $\sim \rm 0.01-10\, mJy$ at GHz frequency, detectable by Jansky Very Large Array, and $\sim 10-100\,\mu\rm Jy$ at $\sim10^{10}-10^{12} \,\rm Hz$ frequencies, detectable by Atacama Large Millimeter/sub-millimter Array. 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.
Highlights
Galaxies grow through accretion and hierarchical mergers
If a floating black holes (BHs) happens to pass through the Milky Way (MW) disc, the non-thermal emission from the accelerated electrons in the bow shock around the BH should produce detectable signals in the radio and infrared bands
The radio flux ∼0.01–10 mJy is detectable by Jansky Very Large Array (JVLA), while the infrared flux ∼0.1−1 μJy is detectable by Hubble Space Telescope (HST) and James Webb Space Telescope (JWST)
Summary
Galaxies grow through accretion and hierarchical mergers. During the final phase of the merger of two central black holes (BHs), anisotropic emission of gravitational waves (GW) kicks the BH remnant with a velocity up to a few hundreds km s−1 (Baker et al 2006; Campanelli et al 2007; Blecha & Loeb 2008). For GW recoils, the typical kick velocity is large enough for the BHs to escape the shallow gravitational potential of low-mass galaxies, but smaller than the escape velocity of the Milky Way (MW) halo This is the case for triple systems as long as the kick velocity is
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