Magnetized Accretion onto and Feedback from Supermassive Black Holes in Elliptical Galaxies

We present three-dimensional magnetohydrodynamic (MHD) simulations of the fueling of supermassive black holes in elliptical galaxies from a turbulent cooling medium on galactic scales, taking M87* as a typical case. We find that the mass accretion rate is increased by a factor of ~ 10 compared with analogous hydrodynamic simulations. The scaling of Ṁ ~ r1/2 roughly holds from ~ 10\,pc to ~ 10⁻³\,pc (~ 10\, rg) with the accretion rate through the event horizon being ~ 10⁻²\, M_\,yr⁻¹. The accretion flow on scales ~ 0.03-3\,kpc takes the form of magnetized filaments. Within ~ 30\,pc, the cold gas circularizes, forming a highly magnetized (β~ 10⁻³) thick disk supported by a primarily toroidal magnetic field. The cold disk is truncated and transitions to a turbulent hot accretion flow at ~0.3\,pc (10³\,rg). There are strong outflows towards the poles driven by the magnetic field. The outflow energy flux increases with smaller accretor size, reaching ~ 3×10⁴³\,erg\,s⁻¹ for rᵢn=8\,rg; this corresponds to a nearly constant energy feedback efficiency of η~0.05-0.1 independent of accretor size. The feedback energy is enough to balance the total cooling of the M87/Virgo hot halo out to ~ 50 kpc. The accreted magnetic flux at small radii is similar to that in magnetically arrested disk models, consistent with the formation of a powerful jet on horizon scales in M87. Our results motivate a subgrid model for accretion in lower-resolution simulations in which the hot gas accretion rate is suppressed relative to the Bondi rate by ~ (10rg/rB)1/2.