FORGE'd in FIRE: Resolving the End of Star Formation and Structure of AGN Accretion Disks from Cosmological Initial Conditions (2309.13115v2)

Abstract: It has recently become possible to zoom-in from cosmological to sub-pc scales in galaxy simulations to follow accretion onto supermassive black holes (SMBHs). However, at some point the approximations used on ISM scales (e.g. optically-thin cooling and stellar-population-integrated star formation [SF] and feedback [FB]) break down. We therefore present the first cosmological radiation-magnetohydrodynamic (RMHD) simulation which self-consistently combines the FIRE physics (relevant on galactic/ISM scales where SF/FB are ensemble-averaged) and STARFORGE physics (relevant on small scales where we track individual (proto)stellar formation and evolution), together with explicit RMHD (including non-ideal MHD and multi-band M1-RHD) which self-consistently treats both optically-thick and thin regimes. This allows us to span scales from ~100 Mpc down to <100 au (~300 Schwarzschild radii) around a SMBH at a time where it accretes as a bright quasar, in a single simulation. We show that accretion rates up to $\sim 10-100\,{\rm M_{\odot}\,yr^{-1}}$ can be sustained into the accretion disk at $\ll 10^{3}\,R_{\rm schw}$, with gravitational torques between stars and gas dominating on sub-kpc scales until star formation is shut down on sub-pc scales by a combination of optical depth to cooling and strong magnetic fields. There is an intermediate-scale, flux-frozen disk which is gravitoturbulent and stabilized by magnetic pressure sustaining strong turbulence and inflow with persistent spiral modes. In this paper we focus on how gas gets into the small-scale disk, and how star formation is efficiently suppressed.

• The paper presents a pioneering simulation bridging galaxy-scale cosmology with small-scale SMBH accretion physics, showing how magnetic fields drive disk dynamics.
• Strong magnetic fields within the accretion disk suppress star formation and are crucial for maintaining efficient mass inflow rates up to 100 M⊙ year⁻¹.
• This suppression of star formation at sub-parsec scales is essential for sustaining quasar-level accretion and implies new feedback models are needed.

Resolving the End of Star Formation and Structure of AGN Accretion Disks

The paper "FORGE'd in FIRE I" by Hopkins et al. presents a pioneering simulation that integrates large-scale cosmological physics with small-scale physics of individual star formation and stellar evolution. This work successfully bridges the gap between galaxy-scale simulations and accretion disk simulations, offering a self-consistent model that spans from a cosmological scale down to the innermost regions around a supermassive black hole (SMBH).

Main Findings

1. Interplay of Magnetic Fields: The paper shows that magnetic fields are crucial on sub-pc scales within the accretion disk. The presence of strong magnetic fields explains the suppression of star formation at these scales and is critical for maintaining efficient torques and high inflow rates into the SMBH, supporting accretion dynamics on these scales.
2. Quasar-Level Inflow Rates: Extending previous studies, the simulation demonstrates that torques, primarily gravitational at larger scales and MHD on smaller scales, can sustain exceptionally high inflow rates of up to 100 M⊙ year⁻¹ into the SMBH. This resolves the challenge of sustaining quasar-level accretion over extended timescales.
3. Suppression of Star Formation: Magnetic torques and increased thermal stability lead to the cessation of star formation within the inner regions, contradicting prior predictions based on unresolved star formation models. This suppression is essential for enabling gas to accumulate and contribute to SMBH accretion in the absence of substantial stellar feedback.

Methodology and Simulation Parameters

The simulation utilizes, for the first time, a hybrid of FIRE's galaxy-scale models and STARFORGE's detailed star formation physics. This synergistic integration enables the simulation to handle the massive dynamic range, from the cosmological environment of approximately 100 Mpc down to scales below 100 au around an SMBH at a time when it accretes as a luminous quasar.

Theoretical Implications and Future Directions

• Magnetic Fields in Accretion Theory: The results emphasize the role of magnetic fields in shaping accretion flows at sub-pc scales. The disk dynamics observed in the simulation—such as strong magnetic fields and reduced star formation—suggest that accretion models must account for these fields to accurately predict SMBH growth.
• Feedback and Galaxy Evolution: The strong suppression of star formation at sub-pc scales implies that SMBH feedback mechanisms might need re-evaluation in current models of galaxy evolution. The reduced stellar activity can significantly alter expected feedback processes.
• Inspiration for Future Simulations: By setting a new standard for dynamic range and included physics, this paper provides a framework for future simulations aiming to explore different initial conditions, such as lower or higher accretion rates pertinent to various classes of AGN and quiescent galaxies like the Milky Way.

Conclusion

This research marks a significant advancement in modeling the coupling between galaxy-scale inflows and SMBH accretion processes. By suppressing star formation in favor of SMBH growth, driven by complex interactions of gravitational, radiative, and magnetic forces, the simulation offers a transformative view into the SMBH's role in cosmic structure formation and evolution. The implications for both theory and observation are vast, suggesting new avenues for understanding the balance of processes governing galaxy dynamics and SMBH growth across cosmic time.