Chaotic cold accretion onto black holes
Gaspari, Ruszowski and Oh, 2013
This paper describes a set of very high resolution, idealized simulations of a supermassive black hole (SMBH) in the central (cD) galaxy of a massive cluster. Since the goal is essentially to challenge the current near-universal use of the Bondi accretion model in simulations, let’s start with the key equation:
Setup Start with a simple NFW dark matter halo, a de Vaucouleurs stellar distribution, gas that traces the gravitational potential and a SMBH, all with masses similar to the observed galaxy group NGC 5044. All of these are modelled in an adaptive mesh grid overlaid on a box of side 52kpc for a total time of 40 Myr. With upto 44 levels of refinement, the simulation resolves sub-parsec scales around the central black hole.
Mass accretion onto the black hole, normalized by Bondi prediction.
Varying physics models
The initial setup thus consists of an ideal gas contracting under gravity. This results in adiabatic, isotropic, smooth accretion of warm gas, identical to the analytic model of Bondi (1952). Indeed, the (numerically) observed accretion in this case exactly matches Bondi’s prediction, since the solid line in Fig 1b is essentially = 1 throughout. What is the dashed line, you ask? Well, that is the accretion rate you would measure if you evaluated the parameters in the Bondi equation as an average over a cluster-centric radius of 1-2 kpc, instead of at the Bondi radius (85pc in this case). In other words, computing gas density and sound speed as an average over large cells overestimates the accretion rate.
The simulations sequentially complicate the physics.
First they add cooling, which occurs due to atomic transitions in the ICM. Observed cluster ICMs tend to be quite enriched in metals, mostly due to ejecta from supernovae. They assume the metallicity of the cluster gas to equal that of the sun, which I thought was generous but is actually supported by Chandra observations (e.g. Vikhlinin et al 2005). The accretion is now boosted by over two orders of magnitude. Of course, the simulation doesn’t model star formation; if it did, a lot of this centrally accreted gas could actually be converted into stars, so we would observe very large star formation rates in over very short time scales in the central galaxies of clusters. We d not.
Next, they add turbulence by “stirring” the gas on large scales (lol 4kpc; this is just about the resolution of our cosmological simulation. It’s so relieving to see that someone is actually probing the smaller regions so that our sub-grid models aren’t full of hot air.)(I’m sorry I can’t help these things.) In reality turbulence can be induced by galaxy motions through the viscous ICM, galaxy-galaxy mergers, AGN and stellar feedback, etc. And this is where things start to look really different.
You now see a slice of the temperature profile of the gas. Adding the metal cooling, as mentioned above, decreases the temperature in the core by over 4 orders of magnitude, but retains the spherical symmetry and isotropy. Adding turbulence creates very cold filaments on very large scales. The accretion is on average as high as in the cooling-only scenario, but with more fluctuations.
Lastly, they consider global heating. In a real cluster, this could come from cosmic rays, AGN feedback, massive stellar feedback, etc. This suppresses the star formation somewhat from the previous case, but the “boost factor” with respect to the Bondi prediction is just under 100 by the end of the simulation. The filaments induced by turbulence are not broken up or significantly heated up.
In summary: accretion of gas onto the supermassive black holes at the centres of galaxy clusters is cold, chaotic and filamentary. Averaged over tens of megayears, the boost factor with respect to the Bondi model is just under 100, compared to the prevalent norm of 100-400.