The project pursued three parallel themes that weave together at critical points
BLACK HOLE ACCRETION
The goal of this theme is to understand the physical processes that allow gas to flow into a black hole and predict how much and in what form the energy is released. Reynolds and collaborators have pursued observational campaigns (mainly in the X-ray band) of accreting supermassive black holes (SMBHs), known as active galactic nuclei (AGN). These observations have revealed signatures of matter outflows from the SMBHs. These data also reveal many of these black holes to be spinning, an important constraint on our understanding of how the black holes have grown. Reynolds published a major review of black hole spin in 2021, and coauthored a major review (with Laha) of the outflows from SMBHs also in 2021.
Theoretical studies of black hole accretion was also a focus of the project. Working with PhD student Sam Turner and PDRAs Mark Avara, Vanessa Lopez-Barquero and Greg Marcel, Reynolds used simplified models to understand the variability and appearance of black hole accretion in terms of the underlying processes. First-principles simulations of black hole accretion allows us to move beyond the restrictions of these reduced models, and has been the focus of work by Reynolds, PhD student Payton Rodman and PDRA Mark Avara. Rodman and Reynolds achieved the demonstration that a disk can spontaneously grow a large-scale magnetic field and create the conditions for powerful jets, but that the properties of these fields/jets are strongly affected by (artificial) conditions of the simulation. This both highlights how generic jets can be and the complexities of capturing them with today’s computational techniques. PDRA Mark Avara has been leading a major effort to understand black hole accretion during the merger of two black holes. He has developed a sophisticated algorithm (PatchWorkMHD) that allows us to capture the complex geometry of a merging black hole system.
PHYSICS OF THE ICM
The intracluster medium (ICM) is a hot/tenuous gas that fills the space between galaxies in a galaxy cluster. This gas is sufficiently hot and tenuous that we must use the framework of “plasma physics” to understand it. Of particular interest is the manner in which heat is transported (conducted) through such plasmas.
Reynolds was one of the principal organisers of a workshop at the Kavli Institute for Theoretical Physics which targeted this class of problems in plasma physics. That workshop highlighted similarities in the theory of heat conduction in such plasmas and the streaming of cosmic rays (highly energetic particles) in normal gas. Exploiting this similarity, Reynolds and his collaborators (principally J. Drake of the University of Maryland, USA) developed a new theoretical framework (Whistler Regulated Magnetohydrodynamics) for understanding heat conduction in these plasmas. Importantly, this framework is amenable to being included in fluid dynamical models of clusters, raising the hopes that we can develop next-generation cluster core models that rigorously include these plasma effects.
A new and unexpected thread of work that has emerged is the connection between ICM plasmas and the search for new physics beyond the standard model (SM) of particle physics.The PI and his collaborators (principally D. Marsh, University of Stockholm) have used new Chandra observations of NGC1275, the central galaxy in the Perseus cluster, to set the most stringent limits to date on the properties of axion-like particles (ALPs). These constraints involve a modelling of the magnetic field in the intracluster medium (ICM), indelibly connecting the MHD of the ICM and these fundamental physics oriented investigations.
FEEDBACK
The threads of the overall project come together in our study of feedback processes in galaxy clusters. Reynolds and PDRA Prakriti PalChoudhury performed a detailed theoretical investigation at the ways that energy injected by a central SMBH can be transported through the ICM, thereby “gently” heating it to prevent catastrophic cooling. It was found that fundamental fluid-dynamic processes — sound waves, internal gravity waves, and turbulence — act in concert to transport energy through the cluster. Towards the end of the project, Reynolds and PalChoudhury took a careful look at the cooling processes, mapping out the mathematical solutions for large-scale spiral cooling fronts. Reynolds and collaborator Sergey Komarov examined the physics of shock waves in the ICM, using a theoretical framework and numerical scheme that extended the usual magnetohydrodynamics to include anisotropic (“channeled”) heat conduction and two-temperature fluids (where the protons and electrons have different temperatures due to weak coupling). On an observational front, Reynolds and Masters Student Chris Bambic conducted a detailed study of feedback processes in the nearby galaxy M84 using a very deep exposure with the Chandra X-ray Observatory.
