Orbital and radiative properties of wandering intermediate-mass black holes in the ASTRID simulation. Issue 3 (1st February 2023)

• Record Type: Journal Article
• Authors: Weller, Emma Jane; Pacucci, Fabio; Ni, Yueying; Chen, Nianyi; Di Matteo, Tiziana; Siwek, Magdalena; Hernquist, Lars
• Is Part Of: Monthly notices of the Royal Astronomical Society. Volume 520:Issue 3(2023)
• Page Start: 3955
• Page End: 3963
• Publication Date: 2023-02-01
• Subjects: accretion, accretion discs -- black hole physics -- methods: numerical -- software: simulations -- galaxies: active; Astronomy -- Periodicals; Periodicals; 520.5
• DOI: 10.1093/mnras/stad347
• Languages: English
• ISSNs: 0035-8711

Abstract

ABSTRACT: Intermediate-Mass Black Holes (IMBHs) of $10^3-10^6\, {\rm M_\odot }$ are commonly found at the centre of dwarf galaxies. Simulations and observations convincingly show that a sizeable population of IMBHs could wander off-centre in 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 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 pericentre of their orbits, with rates 10 −3 − 10 −5 times the Eddington rate and a median accretion duty cycle of $\sim 12{{\ \rm per\ cent}}$ . Their typical spectral energy distribution peaks in the infrared at $\sim 11 \, \mu \rm m$ rest-frame. Assuming a standard value of $10{{\ \rm per\ cent}}$ for the matter-to-energy radiative efficiency, IMBHs reach 2 − 10 keV X-ray luminosities $\gt 10^{37} \, \mathrm{erg\, s^{-1}}$ for $\sim 10{{\ \rm per\ cent}}$ of the time. This luminosity corresponds to fluxes $\gt 10^{-15} \, \mathrm{erg \, s^{-1} \, cm^{-2}}$ within 10 Mpc. They could be challenging to detect because of competing emissions from X-ray binaries and the interstellar medium. X-ray luminosities $\gt 10^{41} \, \mathrm{erg \, s^{-1}}$, in the hyper-luminous X-ray sources (HLXs) regime, are reached by $\sim 7{{\ \rm per\ cent}}$ 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.