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Detectability of Wandering Intermediate-Mass Black Holes in the Milky Way Galaxy from Radio to X-rays

Presentation #114.02 in the session Stellar-Mass and Galactic Black Holes.

Published onJun 29, 2022
Detectability of Wandering Intermediate-Mass Black Holes in the Milky Way Galaxy from Radio to X-rays

Intermediate-mass black holes (IMBHs, 103–6 M), 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 (ADAF) to compute the typical fluxes, in a range of frequencies spanning from radio to X-rays, emitted by IMBHs of 105 M wandering in five realistic, volume-weighted, MW environments. We predict that ~27% of the population of IMBHs can be detected in the X-ray with Chandra, ~34% in the infrared with the Roman Space Telescope, ~49% in the sub-mm with CMB-S4 and ~57% in the radio with ngVLA. We find that the brightest fluxes are emitted by IMBHs passing through molecular clouds or cold neutral medium, and are always detectable in the former environment. We propose criteria to facilitate the selection of candidates in multi-wavelength surveys. Specifically, we compute the X-ray to optical/UV ratio (αox) and the sub-mm to optical ratio, as a function of the accretion rate of the IMBH. We show that at low rates the sub-mm/radio emission of IMBHs is significantly higher than the optical/UV/X-ray emission. Finally, we place robust upper limits on the number N of these objects in the MW: N < 2000 and N < 100, based on our detectability expectations and 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.

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