Intermediate-mass black holes in dwarf galaxies: the case of Holmberg II

In order to constrain the density of intermediate mass black holes (IMBHs) in galaxies, we run smoothed particle hydrodynamics (SPH) simulations of a gas-rich disc dwarf galaxy, where different halo and disc populations of IMBHs are embedded. IMBHs, when passing through dense gas regions, can accrete gas and switch on as X-ray sources. We derive the luminosity distribution of simulated IMBHs, by assuming that they accrete at the Bondi-Hoyle rate. The X-ray distribution of simulated IMBHs has been compared with that of observed sources in the dwarf galaxy Holmberg II, chosen for its richness in gas, its small mass (compared to spiral galaxies) and the accuracy of the available X-ray measurements. Holmberg II also hosts one of the strongest IMBH candidates. From this comparison, we find that the density parameter of disc (halo) IMBHs must be Omega(center dot) less than or similar to 10(-5) Omega(b) (Omega(center dot) less than or similar to 10(-2) Omega(b), where Omega(b) is the density parameter of baryons), for a radiative efficiency 10(-3) and an IMBH mass of 10(4) M-circle dot. These constraints imply that a dwarf galaxy like Holmberg II cannot host more than 1 (1000) disc (halo) 10(4) M-circle dot IMBHs.