We present high-resolution, three-dimensional hydrodynamic simulations of the fueling of supermassive black holes in elliptical galaxies from a turbulent medium on galactic scales, taking M87* as a typical case. The simulations use a new GPU-accelerated version of the Athena++ AMR code, and they span more than six orders of magnitude in radius, reaching scales similar to that of the black hole horizon. The key physical ingredients are radiative cooling and a phenomenological heating model. We find that the accretion flow takes the form of multiphase gas at radii less than about a kpc. The cold gas accretion includes two dynamically distinct stages: the typical disk stage in which the cold gas resides in a rotationally supported disk, and relatively rare chaotic stages (≲10% of the time) in which the cold gas inflows via chaotic streams. Though cold gas accretion dominates the time-averaged accretion rate at intermediate radii, accretion at the smallest radii is dominated by hot virialized gas at most times. The accretion rate scales with radius as M ̇ ∝ r 1 / 2 when hot gas dominates, and we obtain M ̇ ≃ 10 − 4 – 10 − 3 M ⊙ yr − 1 near the event horizon, similar to what is inferred from EHT observations. The orientation of the cold gas disk can differ significantly on different spatial scales. We propose a subgrid model for accretion in lower-resolution simulations in which the hot gas accretion rate is suppressed relative to the Bondi rate by ∼ ( r g / r Bondi ) 1 / 2 . Our results can also provide more realistic initial conditions for simulations of black hole accretion at the event horizon scale.

We investigate low-density accretion flows onto massive black holes (BHs) with masses of ≳105 orbiting around in the outskirts of their host galaxies, performing 3D hydrodynamical simulations. Those wandering BHs are populated via ejection from the galactic nuclei through multibody BH interactions and gravitational wave recoils associated with galaxy and BH coalescences. We find that when a wandering BH is fed with hot and diffuse plasma with density fluctuations, the mass accretion rate is limited at ∼10%–20% of the canonical Bondi–Hoyle–Littleton rate owing to a wide distribution of inflowing angular momentum. We further calculate radiation spectra from radiatively inefficient accretion flows onto the wandering BH using a semianalytical two-temperature disk model and find that the predicted spectra have a peak at the millimeter band, where the Atacama Large Millimeter/submillimeter Array (ALMA) has the highest sensitivity and spatial resolution. Millimeter observations with ALMA and future facilities such as the next-generation Very Large Array (ngVLA) will enable us to hunt for a population of wandering BHs and push the detectable mass limit down to M • ≃ 2 × 107 for massive nearby ellipticals, e.g., M87, and M • ≃ 105 for the Milky Way. This radiation spectral model, combined with numerical simulations, will be applied to give physical interpretations of off-nuclear BHs detected in dwarf galaxies, which may constrain BH seed formation scenarios.

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Short (inner) bars of sub-kiloparsec radius have been hypothesized to be an important mechanism for driving gas inflows to small scales, thus feeding central black holes. Recent numerical simulations have shown that the growth of central black holes in galaxies can destroy short bars, when the black hole reaches a mass of ∼ 0.1% of the total stellar mass of the galaxy. We study N -body simulations of galaxies with single and double bars to track the long-term evolution of the central stellar mass distribution. We find that the destruction of the short bar contributes significantly to the growth of the bulge. The final bulge mass is roughly equal to the sum of the masses of the initial pseudo bulge and short bar. The initially boxy/peanut-shaped bulge of Sérsic index n 1 is transformed into a more massive, compact structure that bears many similarities to a classical bulge, in terms of its morphology (n ≈ 2), kinematics (dispersion-dominated, isotropic), and location on standard scaling relations (Kormendy relation, mass-size relation, and correlations between black hole mass and bulge stellar mass and velocity dispersion). Our proposed channel for forming classical bulges relies solely on the destruction of short bars without any reliance on mergers. We suggest that some of the less massive, less compact classical bulges were formed in this manner.