[Paper Review] Sustained super-Eddington accretion in high-redshift quasars

Alessandro Lupi, G. Quadri+3 more|arXiv (Cornell University)|Dec 13, 2023

Galaxies: Formation, Evolution, Phenomena5 references4 citations

TL;DR

This study demonstrates that sustained super-Eddington accretion—lasting tens of millions of years—can occur in high-redshift quasars within cosmological simulations, enabling rapid black hole growth despite strong feedback. Even with high initial spin and strong jet feedback, the dense, rapidly inflowing gas in massive halos resists expulsion, allowing accretion to rebound quickly after outbursts, explaining the overmassive black holes observed at z > 6.

ABSTRACT

Observations of $z \gtrsim 6$ quasars provide information on the early evolution of the most massive black holes (MBHs) and galaxies. Current observations, able to trace both gas and stellar properties, reveal a population of MBHs that is significantly more massive than expected from the local MBH-stellar mass relation. The population lies on, but mostly above, the relation observed in the nearby Universe. This suggests that these objects grew very rapidly. To explain their presence when the Universe was less than 1~Gyr old and to assess the physical conditions for their rapid growth, we explored whether episodes of accretion above the Eddington limit can occur across cosmic epochs. By employing state-of-the-art high-resolution cosmological zoom-in simulations of a $z\sim 7$ quasar, where different accretion regimes are included consistently, together with their associated radiative and kinetic feedback, we show that super-Eddington phases can be sustained for relatively long timescales (tens of millions of years). This allows the MBH to rapidly grow by up to three orders of magnitude, depending on the strength of the kinetic feedback. We also show by means of a semianalytic calculation that the MBH spin remains moderate and does not take on extremely high values during the super-Eddington phases. This results in a lower feedback efficiency, which may allow the rapid growth required to explain over-massive high-redshift MBHs.

Motivation & Objective

• To investigate whether super-Eddington accretion can be sustained over long timescales (tens of Myr) in high-redshift quasars within a cosmological context.
• To assess the role of radiative and kinetic feedback in regulating black hole growth during super-Eddington phases.
• To examine how MBH spin evolves during super-Eddington accretion and its impact on feedback efficiency.
• To explain the origin of overmassive black holes at z > 6 by testing accretion mechanisms that exceed the Eddington limit.
• To evaluate the robustness of previous idealized simulations by incorporating full cosmological zoom-in simulations with realistic feedback coupling.

Proposed method

• Employed high-resolution cosmological zoom-in simulations of a z ≈ 7 quasar host halo (M_halo ≈ 3×10¹² M☉), including self-consistent accretion physics and feedback.
• Implemented a slim-disc model for super-Eddington accretion, allowing radiation trapping and advective cooling, with radiatively driven outflows and jets.
• Used a fixed black hole spin to isolate the impact of feedback, later testing spin evolution semi-analytically via the Ricarte et al. (2023) model.
• Tracked gas density and temperature evolution around the MBH to assess feedback suppression and recovery timescales.
• Calculated jet efficiency using the Blandford-Znajek mechanism, with MAD-ness determined by the Eddington ratio and empirical scaling (f_Edd/f_c)^α.
• Compared results with previous idealized simulations (e.g., Regan et al. 2019; Massonneau et al. 2023b), focusing on feedback resilience in dense environments.

Experimental results

Research questions

• RQ1 Can super-Eddington accretion phases be sustained for tens of millions of years in a cosmological simulation of a high-redshift quasar?
• RQ2 How does strong jet feedback affect accretion suppression and recovery in gas-rich, high-density galactic nuclei?
• RQ3 What is the evolution of black hole spin during sustained super-Eddington accretion, and how does it affect feedback efficiency?
• RQ4 Why do high-redshift black holes appear overmassive relative to local correlations, and can sustained super-Eddington accretion explain this?
• RQ5 How does the combination of deep potential wells and high inflow rates prevent gas expulsion despite strong feedback?

Key findings

• Super-Eddington accretion phases lasting tens of Myr are sustained in high-redshift quasars due to rapid gas inflow and dense central conditions.
• Despite strong jet feedback, gas density remains high (n_H ≈ 10⁴–10⁵ cm⁻³), and accretion recovers quickly after outbursts due to efficient cooling and inflow.
• The MBH spin decreases rapidly during super-Eddington phases, reaching values around 0.3, which reduces jet efficiency by a factor of ~4 compared to high-spin assumptions.
• The disc MAD-ness averages ~0.5 during super-Eddington phases, indicating a transition toward high-mass accretion disc (HMAD) states rather than standard MAD.
• The combination of high gas density, deep potential well, and short free-fall timescales prevents long-term gas expulsion, enabling sustained growth.
• Even with strong feedback, the system remains resilient, allowing black holes to grow rapidly and become overmassive before feedback becomes self-regulating.