Orbital and Radiative Properties of Wandering Intermediate-Mass Black Holes in the ASTRID Simulation

Abstract

Intermediate-Mass Black Holes (IMBHs), defined as having a mass in the range 10 3 M to 10 6 M , are commonly found at the center of dwarf galaxies. Simulations and observations convincingly show that a sizable population of IMBHs could wander offcenter in local galaxies. We use the cosmological simulation ASTRID to study the orbital and radiative properties of wandering IMBHs in massive galaxies at 𝑧 ∼ 3. We find that this population of undetected black holes has large orbital inclinations (60 • ± 22 • ) with respect to the principal plane of the host. The eccentricity of their orbits is also significant (0.6 ± 0.2) and decreases with time. Wandering IMBHs undergo spikes of accretion activity around the pericenter of their orbits, with rates 10 −3 − 10 −5 times the Eddington rate and a median accretion duty cycle of ∼ 12%. Their typical spectral energy distribution peaks in the infrared at ∼ 11 𝜇m rest frame. The studied IMBHs reach 2 − 10 keV X-ray luminosities > 10 37 erg s −1 for ∼ 10% of the time. This luminosity corresponds to fluxes > 10 −15 erg s −1 cm −2 within 10 Mpc. Two out of the 28 IMBHs studied (∼ 7%) have brief X-ray luminosity spikes > 10 41 erg s −1 , in the hyper-luminous X-ray sources (HLXs) regime. These findings suggest that HLXs are a small subset of the general wandering IMBH population, which is characterized by luminosities 10 3 − 10 4 times fainter. Dedicated surveys with current and future observatories are needed to assess the demographics of this missing population of black holes.