Orbital and Radiative Properties of Wandering IMBHs in the Milky Way and Massive Galaxies at z ~ 3

Intermediate-Mass Black Holes (IMBHs), in a mass range of 10³ - 10⁶ solar masses, are commonly found at the center of dwarf galaxies. Simulations and observations convincingly show that a sizable population of IMBHs could wander off-center in massive galaxies.

We use the cosmological simulation Astrid to study the orbital and radiative properties of wandering IMBHs in massive galaxies at z ~ 3. We show that they have eccentric and tilted orbits, and they only radiate when wandering in the inner volume of the host. Importantly for observations, < 10% of the objects of our sample have X-ray luminosity spikes in the hyper-luminous X-ray regime, suggesting that HLXs are a subset of the wandering population. Dedicated surveys with current and future observatories are needed to assess the demographics of this missing population of black holes.

Using realistic interstellar medium environments, we also present a detectability analysis of wandering IMBHs in the Milky Way Galaxy. This putative population of IMBHs can be detected in a broad part of the electromagnetic spectrum, from radio to X-rays. We develop some selection criteria to identify them in multi-wavelength surveys.