Chaotic cold accretion onto black holes (1301.3130v2)

Abstract: Using 3D AMR simulations, linking the 50 kpc to the sub-pc scales over the course of 40 Myr, we systematically relax the classic Bondi assumptions in a typical galaxy hosting a SMBH. In the realistic scenario, where the hot gas is cooling, while heated and stirred on large scales, the accretion rate is boosted up to two orders of magnitude compared with the Bondi prediction. The cause is the nonlinear growth of thermal instabilities, leading to the condensation of cold clouds and filaments when t_cool/t_ff < 10. Subsonic turbulence of just over 100 km/s (M > 0.2) induces the formation of thermal instabilities, even in the absence of heating, while in the transonic regime turbulent dissipation inhibits their growth (t_turb/t_cool < 1). When heating restores global thermodynamic balance, the formation of the multiphase medium is violent, and the mode of accretion is fully cold and chaotic. The recurrent collisions and tidal forces between clouds, filaments and the central clumpy torus promote angular momentum cancellation, hence boosting accretion. On sub-pc scales the clouds are channelled to the very centre via a funnel. A good approximation to the accretion rate is the cooling rate, which can be used as subgrid model, physically reproducing the boost factor of 100 required by cosmological simulations, while accounting for fluctuations. Chaotic cold accretion may be common in many systems, such as hot galactic halos, groups, and clusters, generating high-velocity clouds and strong variations of the AGN luminosity and jet orientation. In this mode, the black hole can quickly react to the state of the entire host galaxy, leading to efficient self-regulated AGN feedback and the symbiotic Magorrian relation. During phases of overheating, the hot mode becomes the single channel of accretion (with a different cuspy temperature profile), though strongly suppressed by turbulence.

• The paper demonstrates via 3D simulations that "chaotic cold accretion", driven by cooling and turbulence, significantly boosts black hole accretion rates beyond Bondi predictions.
• Radiative cooling alone increases accretion rates by two orders of magnitude, while turbulence helps remove angular momentum barriers by forming cold clouds.
• The chaotic cold accretion model, incorporating heating, offers a framework better explaining rapid black hole growth and observed galaxy-SMBH scaling relations.

Chaotic Cold Accretion onto Black Holes: A Systematic Investigation

This essay provides a critical overview of the paper "Chaotic Cold Accretion onto Black Holes," which examines the modes of accretion onto supermassive black holes (SMBHs) under conditions more representative of astrophysical environments than those described by the classic Bondi theory. The authors employ three-dimensional adaptive mesh refinement (AMR) simulations to evaluate how thermal instability, radiative cooling, turbulence, and heating influence accretion dynamics at scales ranging from sub-parsec to tens of kiloparsec, effectively bridging the gap between SMBH collection and the scales of their host galaxies.

Adiabatic Accretion and the Bondi Paradigm

Initially, the authors validate the classic Bondi model under adiabatic and stratified conditions, observing that the model's predictions hold within a few percent when accretion is computed near the Bondi radius. However, a small bias is introduced when observationally common practice applies the Bondi rate on kiloparsec scales, typically underestimating accretion rates due to the localized decline in density and temperature profiles of massive galaxies’ cores. This bias is modest, underscoring the fundamental reliability of the Bondi approach in static settings even if not at the level necessary to justify boost factors ordered at 100 times or more.

Transition to Radiative Accretion

The introduction of radiative cooling fundamentally shifts the paradigm. The presence of cooling in the simulations drives a two-order magnitude enhancement in the accretion rate over Bondi predictions. This arises because cooling erodes pressure support, instigating rapid cold-phase condensation within sub-kiloparsec zones that are subsequently accreted. Here, the effective sonic radius expands, diminishing the adiabatic inner boundary condition typically assumed by Bondi.

Turbulence and Thermal Instability

Beyond cooling impacts, the simulations capture turbulence-induced non-linear thermal instability as a principal origin of what the authors term "chaotic cold accretion." Even relatively subsonic turbulence with dispersions of 100-300 km/s can seed density fluctuations sufficient for cooling to actuate rapid, cold cloud formation, leading to enhanced, chaotic inflow. The frequent interactions between cold clouds serve to nullify angular momentum barriers that would otherwise limit accretion efficiency. This revelation positions turbulent motions as a central mechanism in such astrophysical settings.

Heating, Thermal Balance, and Multiphasic Media

When accounting for spatially-distributed heating, the modeling shifts to conditions more reflective of large cosmic structures, such as galaxy clusters. Heating generally matches cooling on large scales but permits local instabilities consistently where $t_{\rm cool}/t_{\rm ff}$ falls below a critical value near ten. This set of simulations exemplifies a chaotic accumulation of cold clouds drawn towards the black hole, bolstered by heating-induced pressure fluctuations and nonlinear exponential growth of the instabilities. The presence of ambient heating moderates cooling effectiveness, underpinning a chaotic but steady-state solution punctuated by stochastic accretion fluctuations.

Broader Implications and Theoretical Considerations

Practically, the insights on chaotic cold accretion provide a framework that transitions beyond the limitations set by traditional Bondi accretion mechanics. This framework can better explain rapid black hole growth across cosmic epochs, observed SMBH-galaxy scaling relations, and the internally coherent symbiotic relationship between galaxies and their central black holes. The enhanced accretion rates inferred from chaotic processes address mismatches in cosmological simulation outputs relative to empirical data, revealing that traditional formulas fail to capture the dynamism imposed by multiphase gas dynamics.

Conclusively, this paper posits Chaotic Cold Accretion as a potent lens through which astrophysical accretion flows should be reconsidered, endowed with mechanisms more resonant with empirical data. The model prompts an evolution in how texture and turbulence in individual environments contribute to accretion dynamics and broader cosmological phenomena, beckoning further paper into more nuanced sub-grid physics in simulations of galaxy formation and evolution.