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

Abstract: 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 $\sim 10$ compared with analogous hydrodynamic simulations. The scaling of $\dot{M} \sim r^{1/2}$ roughly holds from $\sim 10\,\mathrm{pc}$ to $\sim 10^{-3}\,\mathrm{pc}$ ($\sim 10\, r_\mathrm{g}$) with the accretion rate through the event horizon being $\sim 10^{-2}\, M_\odot\,\mathrm{yr^{-1}}$. The accretion flow on scales $\sim 0.03-3\,\mathrm{kpc}$ takes the form of magnetized filaments. Within $\sim 30\,\mathrm{pc}$, the cold gas circularizes, forming a highly magnetized ($\beta\sim 10^{-3}$) thick disk supported by a primarily toroidal magnetic field. The cold disk is truncated and transitions to a turbulent hot accretion flow at $\sim0.3\,\mathrm{pc}$ ($10^3\,r_\mathrm{g}$). There are strong outflows towards the poles driven by the magnetic field. The outflow energy flux increases with smaller accretor size, reaching $\sim 3\times10^{43}\,\mathrm{erg\,s^{-1}}$ for $r_\mathrm{in}=8\,r_\mathrm{g}$; this corresponds to a nearly constant energy feedback efficiency of $\eta\sim0.05-0.1$ independent of accretor size. The feedback energy is enough to balance the total cooling of the M87/Virgo hot halo out to $\sim 50$ 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 $\sim (10r_\mathrm{g}/r_\mathrm{B})^{1/2}$.

• The paper demonstrates that magnetic fields enable approximately 10x higher SMBH accretion rates by efficiently transporting angular momentum.
• The paper reveals distinct accretion flow regimes from chaotic kiloparsec-scale inflows to sub-parsec magnetically arrested disks.
• The paper finds that magnetically driven polar outflows carry significant energy, potentially balancing halo cooling and regulating feedback.

Overview of Magnetized Accretion onto Supermassive Black Holes in Elliptical Galaxies

The paper presents a comprehensive study of magnetohydrodynamic (MHD) simulations focusing on the accretion of matter onto supermassive black holes (SMBHs) in elliptical galaxies, specifically using M87* as a prototypical example. The research is centered on understanding how magnetized accretion processes influence the feeding and feedback mechanisms of SMBHs, particularly within the complex astrophysical environments of elliptical galaxies.

Key Findings

1. Enhanced Accretion Rates: The MHD simulations demonstrate that the presence of magnetic fields significantly enhances the mass accretion rate by approximately an order of magnitude compared to purely hydrodynamic simulations. This increase is attributed to the efficient angular momentum transport facilitated by magnetic stresses.
2. Accretion Flow Structures: The simulations reveal distinct accretion flow structures across various scales:
   • On kiloparsec scales, the accretion is characterized by chaotic inflows along magnetized filaments.
   • On parsec scales, a highly magnetized and thick disk forms, supported primarily by toroidal magnetic fields.
   • Within sub-parsec scales, the accretion transitions into a hot, turbulent flow consistent with a magnetically arrested disk (MAD) state.
3. Outflows and Feedback: Strong outflows are observed, driven by magnetic forces towards the poles of the SMBH. These outflows exhibit substantial energy flux, which increases as the accretor size decreases, maintaining a nearly constant feedback efficiency. This energy output is sufficient to potentially balance the cooling of the surrounding hot halo, highlighting the significance of magnetic fields in regulating SMBH environments.
4. Subgrid Model for Simulations: The study proposes an updated subgrid model for accretion in large-scale simulations, suggesting that the hot gas accretion rate is suppressed relative to the Bondi rate, following a particular scaling law. This improvement is crucial for modeling SMBH growth and feedback in simulations lacking the resolution to directly capture these small-scale processes.

Implications and Future Directions

The findings of this study have significant implications for theoretical models of SMBH growth and feedback mechanisms in galaxies. The enhanced understanding of magnetized accretion processes provides a more nuanced framework for interpreting observations of active galactic nuclei (AGN) and their environments. The demonstration that magnetic fields can drastically alter accretion dynamics highlights the need to incorporate magnetized processes into broader models of galaxy evolution.

Looking forward, future research could build on these results by exploring the impacts of magnetic fields in different galactic morphologies and environments. Additionally, the role of varying black hole spin configurations on accretion dynamics warrants further investigation. These studies could further refine our understanding of the complex layers of interaction between SMBHs and their host galaxies, aiding in the development of more sophisticated and predictive cosmological simulations.