Constraints on Galactic intermediate mass black holes

Intermediate mass black holes (IMBHs; 10^1.3 to 10⁵M_solar) are thought to form as relics of Population III stars or from the runaway collapse of stars in young clusters; their number and very existence are uncertain. We ran N-body simulations of Galactic IMBHs, modelling them as a halo population distributed according to a Navarro, Frenk & White (NFW) or a more concentrated Diemand, Madau & Moore (DMM) density profile. As IMBHs pass through Galactic molecular/atomic hydrogen regions, they accrete gas, thus becoming X-ray sources. We constrain the density of Galactic IMBHs, Ω_•, by comparing the distribution of simulated X-ray sources with the observed one. From the null detections of Milky Way ultra-luminous X-ray sources, and from a comparison of simulations with unidentified sources in the Imager on Board the International Gamma-Ray Astrophysics Laboratory (INTEGRAL) Satellite/INTEGRAL Soft Gamma-Ray Imager (IBIS/ISGRI) catalogue, we find a strong upper limit Ω_•<= 10^-2Ω_b(<=10^-1Ω_b) for a DMM (NFW) profile, if IMBHs accrete via advection-dominated accretion flow (ADAF) discs. Slightly stronger constraints (Ω_•<= 10^-3Ω_b for a DMM profile; Ω_•<= 10^-2Ω_b for a NFW profile) can be derived if IMBHs accrete with higher efficiency, such as by forming thin accretion discs. Although not very tight, such constraints are the most stringent ones derived so far in the literature.