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

Abstract: Intermediate-Mass Black Holes (IMBHs) of $10^3-10^6 \, M_\odot$ 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 galaxies. We use the cosmological simulation ASTRID to study the orbital and radiative properties of wandering IMBHs in massive galaxies at $z\sim3$. We find that this population of black holes has large orbital inclinations ($60^\circ\pm22^\circ$) with respect to the principal plane of the host. The eccentricity of their orbits is also significant ($0.6\pm0.2$) 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 $\sim 12\%$. Their typical spectral energy distribution peaks in the infrared at $\sim 11 \, \mu \rm m$ rest-frame. Assuming a standard value of $10\%$ for the matter-to-energy radiative efficiency, IMBHs reach $2-10$ keV X-ray luminosities $>10^{37} \, \mathrm{erg\,s^{-1}}$ for $\sim10\%$ of the time. This luminosity corresponds to fluxes $>10^{-15} \, \mathrm{erg \, s^{-1} \, cm^{-2}}$ within $10$ Mpc. They could be challenging to detect because of competing emissions from X-ray binaries and the interstellar medium. X-ray luminosities $> 10^{41} \, \mathrm{erg \, s^{-1}}$, in the hyper-luminous X-ray sources (HLXs) regime, are reached by $\sim 7\%$ of the IMBHs. These findings suggest that HLXs are a small subset of the wandering IMBH population, which is characterized by luminosities $10^3-10^4$ times fainter. Dedicated surveys are needed to assess the demographics of this missing population of black holes.