So excited about this fall! I get to implement a new subgrid model for accretion onto, and feedback from, active galactic nuclei (AGN) at the centres of clusters in a cosmological simulation we develop here at Yale.
Developing subgrid models (and numerical approximations in general) is an art as much as a science. On the one hand, you need to reproduce large-scale, observable phenomena, like star formation histories, stellar masses, stellar and metallicities, and dark and visible substructure. On the other, you want your input parameters to be motivated by plausible underlying physics.
Fig 1 from Urry and Padovani, 1995
AGN are a pesky beast. They consist of a supermassive black hole (SMBH) at the core, or nucleus, of a galaxy, surrounded by a hot disk of infalling matter. Since the material in the disk is unlikely to have falling in on a radial trajectory, it settles in a rotating disk around the black hole. Friction between layers of the disk heat it up to tens of millions of Kelvin, so that if you catch an AGN at the correct angle, it is bright in the X-ray. More often than not, due to geometric reasons, you’ll end up seeing dust-obscured AGN, which are bright in the radio. That’s a two-line summary of the Unified Model of AGN.
This obscuration, combined with the final parsec problem, is the main reason AGN are so pesky. Stuff falls onto black hole, aggravates it, isothermal heat ejections and mechanical jets arise! But what sort of stuff can fall into the black hole? Why does that cause it to spit stuff out? How exactly does it eject this energy? How far does the energy travel, and how does it interact with gas on its way (more specifically, the Intra-Cluster Medium or the ICM)?
There has been a sea of observations and theoretical models on this topic in the last few decades, and I’m just starting to dip my toes in it. Here’s a summary of the papers I’ll review in the next months.
- How is energy transported within an accretion disk? How do the viscosity, density and temperature of the gas in the accretion disk determine whether energy transport is dominated by radiation, advection or convection? What does each of these processes look like? Advection-Dominated Accretion around Black Holes – Narayan, Mahadevan and Quataert, 1998
- How exactly does the gas fall onto the black hole? How cold does it have to be? Does this depend on whether the gas is in filaments or clouds, and how those may be oriented? Growing supermassive black holes by chaotic accretion – Gaspari et al, 2013
- The simulation I work with extracts clusters of galaxies from a cosmological box. This captures things idealized/isolated cluster simulations cannot, like smooth accretion of gas from filaments and mergers of clusters. Accretion during the merger of supermassive black holes – Armitage and Natarajan, 2002
- Several self-regulating mechanisms have resulted in tight relations between galaxies and the supermassive black holes that live in their centres. Accreting supermassive black holes in the COSMOS field and the connection to their host galaxies – Bongiorno et al, 2012.