[Paper Review] FORGE'd in FIRE: Resolving the End of Star Formation and Structure of AGN Accretion Disks from Cosmological Initial Conditions

Philip F. Hopkins, Michael Y Grudić+9 more|arXiv (Cornell University)|Sep 22, 2023

Astrophysical Phenomena and Observations4 citations

TL;DR

This study presents the first cosmological radiation-magnetohydrodynamic simulation that self-consistently couples FIRE-scale galactic physics with STARFORGE-scale stellar formation and feedback, resolving down to <100 au around a supermassive black hole. It reveals that gravitational torques and strong magnetic fields suppress star formation within sub-parsec scales, enabling sustained accretion rates of 10–100 M⊙ yr⁻¹ into the disk via a gravitoturbulent, flux-frozen structure stabilized by magnetic pressure.

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 &lt;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 $\sim 10-100\,{ m M_{\odot}\,yr^{-1}}$ can be sustained into the accretion disk at $\ll 10^{3}\,R_{ m schw}$, 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.

Motivation & Objective

• To resolve the transition from cosmological to sub-pc scales in galaxy simulations, where standard ISM-scale approximations break down.
• To investigate the physical mechanisms enabling sustained high accretion rates onto supermassive black holes during quasar phases.
• To understand the suppression of star formation in the immediate vicinity of active galactic nuclei due to extreme conditions.
• To provide self-consistent initial and boundary conditions for future high-resolution accretion disk simulations.

Proposed method

• A zoom-in cosmological simulation spanning from ~100 Mpc down to <100 au around a supermassive black hole, using adaptive mesh refinement.
• Self-consistent coupling of FIRE physics (on galactic/ISM scales) with STARFORGE physics (on stellar/sub-pc scales) for star formation and feedback.
• Incorporation of non-ideal magnetohydrodynamics (NIMHD), including ambipolar diffusion, Ohmic resistivity, and anisotropic conduction, to model optically thick and thin regimes.
• Use of a multi-band M1 radiation hydrodynamics solver with detailed thermochemistry, dust opacities, and cosmic ray transport.
• Implementation of a live supermassive black hole sink particle with accretion tracked down to 300 Schwarzschild radii.
• Adaptive refinement to resolve gas and stellar structures down to <1 M⊙, with consistent gravity and hydrodynamics solvers.

Experimental results

Research questions

• RQ1 How does gas transport and accretion proceed from galactic scales down to the innermost regions of a quasar-hosting galaxy?
• RQ2 What physical mechanisms suppress star formation in the sub-parsec environment around a supermassive black hole?
• RQ3 How do magnetic fields and gravitational torques shape the structure and stability of the accretion disk at sub-pc scales?
• RQ4 What are the self-consistent accretion rates and disk properties when both large-scale galactic physics and small-scale stellar feedback are resolved?
• RQ5 How do radiation, magnetic fields, and cooling opacity jointly regulate the transition from turbulent, star-forming gas to a stable, accreting disk?

Key findings

• Accretion rates of 10–100 M⊙ yr⁻¹ are sustained into the accretion disk at distances <10³ Schwarzschild radii, despite extreme conditions.
• Star formation is almost completely suppressed within sub-parsec scales due to high optical depth to cooling and strong toroidal magnetic fields raising the magnetic critical mass above the disk mass.
• A gravitoturbulent, flux-frozen disk forms at intermediate scales, stabilized by magnetic pressure and sustaining persistent spiral modes and inflow.
• Gravitational torques between stars and gas dominate on sub-kpc scales and are the primary driver of angular momentum transport prior to star formation shutdown.
• The disk structure is stabilized by magnetic pressure, with non-ideal MHD effects playing a crucial role in maintaining turbulence and inflow at small scales.
• The simulation provides predictive inner boundary conditions for future accretion disk simulations, revealing a previously unexplored regime of strongly magnetized, turbulent accretion disks.