The detection of Intermediate-Mass Black Holes (IMBHs) in dwarf galaxies is crucial to closing the gap in the wide mass distribution of black holes (∼3 M⊙ to ∼5 × 1010 M⊙). IMBHs originally located at the center of dwarfs that later collide with the Milky Way (MW) could be wandering, undetected, in our Galaxy. We used TNG50, the highest-resolution run of the IllustrisTNG project, to study the kinematics and dynamics of star clusters, in the appropriate mass range, acting as IMBH proxies in a MW analog galaxy. We showed that $\sim 87\%$ of our studied IMBHs drift inward. The radial velocity of these sinking IMBHs has a median magnitude of ∼0.44 ckpc h−1 Gyr−1 and no dependence on the black hole mass. The central $1 \, \rm ckpc \, h^{-1}$ has the highest number density of IMBHs in the galaxy. A physical toy model with linear drag forces was developed to explain the orbital circularization with time. These findings constrain the spatial distribution of IMBHs, suggesting that future searches should focus on the central regions of the Galaxy. Additionally, we found that the 3D velocity distribution of IMBHs with respect to the galactic center has a mean of ∼180 km s−1 and larger variance with decreasing radius. Remarkably, the velocity distribution relative to the local gas shows significantly lower values, with a mean of ∼88 km s−1. These results are instrumental for predicting the accretion and radiation properties of IMBHs, facilitating their detection with future surveys.
Recent dynamical measurements indicate the presence of a central SMBH with mass ∼3 × 106 M⊙ in the dwarf galaxy Leo I, placing the system ∼50 times above the standard, local MBH − M⋆ relation. While a few over-massive central SMBHs are reported in nearby isolated galaxies, this is the first detected in a Milky Way satellite. We used the ASTRID and Illustris TNG50 LCDM cosmological simulations to investigate the assembly history of galaxies hosting over-massive SMBHs. We estimate that, at the stellar mass of Leo I, $\sim 15~{{\ \rm per\ cent}}$ of galaxies above the MBH − M⋆ relation lie >10 times above it. Leo I-like systems are rare but exist in LCDM simulations: they occur in $\sim 0.005~{{\ \rm per\ cent}}$ of all over-massive systems. Examining the properties of simulated galaxies harboring over-massive central SMBHs, we find that: (i) stars assemble more slowly in galaxies above the MBH − M⋆ relation; (ii) the gas fraction in these galaxies experiences a significantly steeper decline over time; and (iii) $>95~{{\ \rm per\ cent}}$ of satellite host galaxies in over-dense regions are located above the MBH − M⋆ relation. This suggests that massive satellite infall and consequent tidal stripping in a group/dense environment can drive systems away from the MBH − M⋆ relation, causing them to become over-massive. As the merging histories of over-massive and under-massive systems do not differ, we conclude that additional environmental effects, such as being in overdense regions, must play a crucial role. In the high-z Universe, central over-massive SMBHs are a signature of heavy black hole seeds; we demonstrate, in contrast, that low-z over-massive systems result from complex environmental interactions.
Intermediate-Mass Black Holes (IMBHs) of 103 − 106 M⊙ 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 ∼ 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 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 >1037 erg s−1 for $\sim 10\%$ of the time. This luminosity corresponds to fluxes >10−15 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 >1041 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 103 − 104 times fainter. Dedicated surveys are needed to assess the demographics of this missing population of black holes.
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.
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