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

TL;DR

The study presents a novel cosmological RMHD simulation that unifies FIRE galactic/ISM physics with STARFORGE star formation physics in a single framework, achieving a zoom from ~100 Mpc to <100 au around a SMBH during a quasar phase. By coupling non-ideal MHD, multi-band radiation, thermo-chemistry, and explicit star formation prescriptions, the authors demonstrate sustained inflow of ~$10-100$ $M_{ m ext{sun}}$ yr⁻¹ down to sub-pc scales, while star formation is dramatically suppressed interior to ∼0.1 pc due to optical-depth effects and strong toroidal magnetic fields; the inner disk becomes a flux-frozen, magnetized structure where MHD torques drive accretion. Gravitational torques dominate at larger radii, whereas Maxwell/Reynolds stresses take over in the inner regions, enabling persistent high-rate fueling of the SMBH. The results underscore the importance of magnetic fields and detailed RMHD-chemistry in shaping accretion disk structure and star formation in quasar environments, providing self-consistent boundary conditions for smaller-scale disk models and informing future IMF and observable predictions. The work also acknowledges limitations, notably analyzing a single case and the short duration after hyper-refinement, and outlines plans for Papers II and III to dissect disk physics and the IMF in the circumnuclear region.

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.

Figures (19)

• Figure 1: Series of images of the projected gas density in our simulation (§ \ref{['sec:methods:ics']}) at one moment in time at redshift $z\approx 4$ typical of when we analyze it. Color encodes surface density increasing black-to-white on a logarithmic scale (each panel rescaled owing to the different dynamic range) -- a median pixel in the largest-scale panel ( top-left) has column $N_{H} \sim 10^{19}\,{\rm cm^{-2}}$ (density $n_{H} \sim 10^{-5}\,{\rm cm^{-3}}$), while in the smallest-scale panel $N_{H} \sim 10^{27}\,{\rm cm^{-2}}$ ($n_{H} \sim 10^{12}\,{\rm cm^{-3}}$). We see structure on all scales, with a chaotic, cold, disordered morphology on most scales until an ordered disk forms from capture of gas from a passage of a giant molecular cloud complex (itself triggered by an ongoing galaxy merger in the rapidly-accreting proto-galaxy), forming the accretion disk at $\lesssim 0.1\,$pc.
• Figure 2: As Fig. \ref{['fig:images.faceon.stylized']}, but tiling the images so more structure can be seen and identifying each with the heuristic label appropriate to the range of scales shown, per § \ref{['sec:scales']}. In order, each image zooms in by a factor of $10$ around the previous image, with side-length $L = (1000,\,100,\,10)\,{\rm kpc}$ ( top), $(1000,\,100,\,10)$ pc ( middle), $(1,\,0.1,\,0.01)$ pc ( bottom). The projection here is chosen to be face-on to the innermost central disk. Note the "hole" in the latter inside $r \lesssim 80\,$au is caused by our inner accretion boundary (dashed circle).
• Figure 3: As Fig. \ref{['fig:images.faceon']}, but in stars. The chaotic merger morphology at a few kpc, and clumpy, highly asymmetric stellar morphology driving gravitational torques on the gas is evident on all scales. In most panels we show a continuous projection of stellar density, but in the last panel this breaks down (the inter-stellar separation is no longer much smaller than a pixel) so we show individual O-stars. We do not show images at $\ll 1\,$pc because there is a negligible stellar mass compared to the gas on these scales.
• Figure 4: Illustration of the time evolution of the main galaxy in our simulation before our hyper-refinement. We plot the galaxy-integrated SFR $\dot{M}_{\ast}$, and the sub-grid estimated BH accretion rate (BHAR) from the model -- which scales approximately as $\dot{M}_{\rm subgrid} \sim \eta\,M_{\rm gas}\,\Omega$ at the low-resolution limit of $\sim 10-100\,$pc before hyper-refinement is turned on -- as a function of time prior to the time of refinement (at $z\sim 4.4$). The inset shows the resolved gas inflow rate into the central sink resolution around the SMBH of $<80\,$au, as a function of time in units of the dynamical time $t_{\rm dyn}\equiv 1/\Omega$ at this resolution scale ($\sim 80\,$au), over the final $\sim 1500\,$yr of our simulation duration (well after it reaches the maximum refinement level everywhere). Though this is a very short relative timescale (compared to the order Hubble time evolution on large scales), we see that the inflow rate into the inner accretion disk is quite stable over tens of thousands of dynamical times in the center, for a given set of conditions at larger radii. We also see extremely high inflow rates, as expected based on the high nuclear gas masses and densities in the "parent" simulation which motivated the choice of this particular moment in time to "zoom in."
• Figure 5: Images of the gas ( left) and stars ( right) with different scales and their approximate naming label conventions from § \ref{['sec:scales']} shown. The images show BH-centric radius $r$ increasing from bottom-to-top (the vertical axis) on a logarithmic scale as labeled. The horizontal axis shows $\cos{\theta} \equiv z/r$ from $-1$ to $+1$ (defined so $z=0$ corresponds to the midplane of the inner disk), in a wedge of azimuthal opening angle $\sin{\phi} < 0.3$. For gas ( left) colors denote different phases $T<10^{3}\,$K ( green), $10^{3}<T<10^{4}$ K ( yellow), $10^{4}<T<10^{5}\,$K ( magenta), $10^{5}<T<10^{6}\,$K ( purple), $T>10^{6}\,$K ( cyan). We see other galaxies on IGM scales, the virialized CGM with accretion in warm clumps/filaments, the highly clumpy/inhomogeneous/asymmetric and multi-phase structure in the galaxy and (thermally colder, primarily atomic+molecular) galaxy nucleus, settling into the more ordered (but still visibly thick and turbulent) non-star forming disk and BH accretion disk on sub-pc scales.