[Paper Review] Cosmological simulations of quasar fueling to sub-parsec scales using Lagrangian hyper-refinement

Daniel Anglés‐Alcázar, Eliot Quataert+8 more|arXiv (Cornell University)|Aug 27, 2020

Galaxies: Formation, Evolution, Phenomena377 references102 citations

TL;DR

This study presents the first cosmological hydrodynamic simulations resolving gas inflow to sub-parsec scales (0.1 pc) around a quasar-mass black hole using Lagrangian hyper-refinement. It reveals that inflow rates reach ~6 M⊙ yr⁻¹ at z ≈ 2, driven by gravitational torques from multi-scale stellar non-axisymmetries, with highly variable duty cycles due to hot cavity formation, and shows that accreting gas forms misaligned, rotationally supported obscuring structures.

ABSTRACT

We present cosmological hydrodynamic simulations of a quasar-mass halo ($M_{ m halo} \approx 10^{12.5}\,{ m M}_{\odot}$ at z=2) that for the first time resolve gas transport down to the inner 0.1 pc surrounding the central massive black hole. We model a multi-phase interstellar medium including stellar feedback by supernovae, stellar winds, and radiation, and a hyper-Lagrangian refinement technique increasing the resolution dynamically approaching the black hole. We do not include black hole feedback. We show that the sub-pc inflow rate (1) can reach ~6 M$_{\odot}$yr$^{-1}$ roughly in steady state during the epoch of peak nuclear gas density (z~2), sufficient to power a luminous quasar, (2) is highly time variable in the pre-quasar phase, spanning 0.001-10 M$_{\odot}$yr$^{-1}$ on Myr timescales, and (3) is limited to short (~2 Myr) active phases (0.01-0.1 M$_{\odot}$yr$^{-1}$) followed by longer periods of inactivity at lower nuclear gas density and late times (z~1), owing to the formation of a hot central cavity. Inflowing gas is primarily cool, rotational support dominates over turbulence and thermal pressure, and star formation can consume as much gas as provided by inflows across 1 pc - 10 kpc. Gravitational torques from multi-scale stellar non-axisymmetries dominate angular momentum transport over gas self-torquing and pressure gradients, with accretion weakly dependent on black hole mass. Sub-pc inflow rates correlate with nuclear (but decouple from global) star formation and can exceed the Eddington rate by x10. The black hole can move ~10 pc from the galaxy center on ~0.1 Myr. Accreting gas forms pc-scale, rotationally supported, obscuring structures often misaligned with the galaxy-scale disk. These simulations open a new avenue to investigate black hole-galaxy co-evolution.

Motivation & Objective

• To simulate gas inflow to sub-parsec scales around a quasar-mass black hole in a cosmological context.
• To investigate the physical mechanisms driving gas transport from kpc to sub-pc scales without black hole feedback.
• To assess the role of stellar feedback, multi-phase ISM, and gravitational torques in regulating black hole fueling.
• To test the robustness of results to numerical resolution and gravitational softening schemes.
• To explore the formation of obscuring structures and black hole motion on short timescales.

Proposed method

• Utilizes cosmological hydrodynamic simulations with Lagrangian hyper-refinement to dynamically increase resolution toward the black hole.
• Models a multi-phase interstellar medium with stellar feedback from supernovae, winds, and radiation using the FIRE-2 framework.
• Employs adaptive gravitational softening and stellar splitting to improve force resolution and numerical convergence.
• Tracks gas inflow rates, angular momentum transport, and star formation across 1 pc–10 kpc scales.
• Uses a hybrid accretion model that switches from sub-grid to resolved particle capture as resolution increases.
• Performs convergence tests across multiple resolution levels and numerical implementations to validate results.

Experimental results

Research questions

• RQ1 What are the time-averaged and time-variable gas inflow rates to the inner 0.1 pc of a quasar-mass black hole at z ≈ 2?
• RQ2 What physical mechanisms dominate angular momentum transport in the innermost regions of the galaxy?
• RQ3 How does the formation of a hot central cavity affect the duty cycle of black hole accretion?
• RQ4 To what extent do obscuring structures form in the sub-pc region, and are they aligned with the galaxy-scale disk?
• RQ5 How sensitive are the inflow rates and black hole growth to numerical resolution and gravitational softening?

Key findings

• Sub-pc gas inflow rates reach ~6 M⊙ yr⁻¹ in steady state during peak nuclear gas density at z ≈ 2, sufficient to power a luminous quasar.
• Inflow rates are highly time-variable in the pre-quasar phase, spanning 0.001–10 M⊙ yr⁻¹ on Myr timescales.
• Accretion is limited to short active phases (~2 Myr) of 0.01–0.1 M⊙ yr⁻¹, followed by inactivity due to the formation of a hot, low-density central cavity.
• Gravitational torques from multi-scale stellar non-axisymmetries dominate angular momentum transport, with accretion weakly dependent on black hole mass.
• Inflow rates can exceed the Eddington rate by up to a factor of 10, and the black hole can move ~10 pc from the galactic center on ~0.1 Myr timescales.
• Accreting gas forms pc-scale, rotationally supported, obscuring structures that are often misaligned with the galaxy-scale disk.