The impact of magnetic fields on gas accretion onto supermassive black holes and AGN feedback: the next frontier of galaxy formation cosmological simulations

Supervisors: Julien Devriendt, Adrianne Slyz

It is now well established that the main mechanism to fuel super massive black holes (SMBH) around which a sub-parsec sized accretion disk is spinning, is the magneto-rotational instability (Balbus & Hawley 1991). There also exists compelling observational evidence that SMBHs are ubiquitous and play an important role in regulating galaxy properties (mass, size, morphology) through extremely energetic AGN feedback events. 

However, cosmological galaxy formation simulations, by and large, ignore the effect of magnetic fields.

Presumably this failure reflects the fact that star formation and stellar feedback, and SMBH formation, 

accretion and feedbacktake place on extremely small, sub-galactic scales, making it a tremendous challenge for simulations to model them with reasonable accuracy whilst resolving the galaxy larger scale environment at the same time. Building on previous work within our group (Beckmann, Devriendt, Slyz 2018, Beckmann, Slyz & Devriendt 2019, Martin-Alvarez et al 2018, Katz, Martin-Alvarez et al 2019, Martin-Alvarez et al 2020) we propose to develop a fully magnetised implementation of SMBHs and AGN feedback in an explicit cosmological context.

The DPhil project will have several steps starting from revisiting the classic Bondi-Hoyle-Lyttleton accretion model onto a point source to magnetize it, placing the black hole within an isolated galactic disk, adding AGN feedback to it before finally moving to the cosmological environment.

The student will also be given the opportunity to develop their own model for galaxy synchrotron emission based on the post-processing of these galactic and cosmological MHD simulations, in a view to produce realistic mock observational data for the coming  Square Kilometer Array instrument and its precursors (in interaction with the radio astronomy observational group at Oxford centred around Prof. Jarvis).

Although no prior knowledge of numerics is required to carry out the project, a strong taste for theoretical physics and the numerical implementation of physical problems is mandatory. 

Black hole mergers in active galactic nuclei

Supervisor: Bence Kocsis

The recent discovery of gravitational waves opened new horizons for understanding the Universe. The measurements have unveiled an abundant population of stellar mass black hole mergers. The great challenge is to understand the possible astrophysical mechanisms that may lead to mergers. The existing theoretical models of their astrophysical origin are currently either highly incomplete or in tension with data (Barack+ 2018).

An interesting possibility is that stellar mass black hole mergers take place around supermassive black holes in the centers of galaxies. In these regions, the number density of stars and stellar black holes is up to a billion times higher than in the Solar neighborhood. Gas in the vicinity of a supermassive black hole collapses to a thin disk and rotates with nearly the speed of light, heats up, and releases a super-bright source of electromagnetic radiation. Such "quasars" often outshine all the stars of the host galaxy combined.

In this project, the student will work with Prof. Bence Kocsis to build a comprehensive model of quasars accounting for the population of stellar mass black holes which surround this region. The black holes twist and warp the disk gravitationally and contribute to heating the disk. The gas in turn slows down the black holes, and causes them to settle into the disk, and catalyzes the formation of binaries, ultimately leading to mergers. We study the evolution of these systems, determine the rate of black hole mergers in these environments, and examine the distinguishing features of electromagnetic and gravitational waves emitted by this source population.

Links to further reading:

Abbott R. et al., 2021, ApJL 913, 7

Barack L. et al., 2019, Classical and Quantum Gravity, Volume 36, Issue 14, article id. 143001

Bartos, Kocsis, Haiman, Marka,  2017, ApJ, 835, 165

Tagawa, Haiman, Kocsis, 2020, ApJ, 898, 25



The response of stellar discs to perturbations

Supervisor: John Magorrian

The starting point for understanding the dynamical structure of self-gravitating stellar discs is usually the construction of an axisymmetrized equilibrium model.  Real stellar discs are never axisymmetric though and so the next step is to apply the methods of perturbation theory to calculate both the qualitative and quantitive response of such models to the various types of noise experienced by real discs.  Most investigations to date have focused on the frequency-dependent response of the disc, but this project will adopt an explicitly time-dependent approach, which also makes comparison with $N$-body models more straightforward.  Its goals are to investigate various ways of treating (i) orbital resonances and (ii) 3d structure applied to either the stellar kinematics of the solar neighbourhood in our own Galactic disc or to the eccentric disc around the supermassive black hole in M31.

Free-energy flows in turbulent astrophysical plasmas

Supervisors: Michael Barnes, Alexander Schekochihin

In magnetised astrophysical plasmas, there is a turbulent cascade of electromagnetic fluctuations carrying free energy from large to small scales. The energy is typically extracted from large-scale sources (e.g., in the solar wind, the violent activity in the Sun’s corona; in accretion discs, the Keplerian shear flow; in galaxy clusters, outbursts from active galactic nuclei) and deposited into heat – the internal energy of ions and electrons. In order for this dissipation of energy to happen, the energy must reach small scales – in weakly collisional plasmas, these are small scales in the 6D kinetic phase space, i.e., what emerges are large spatial gradients of electric and magnetic fields and large gradients of the particle distribution functions with respect to velocities.

This prompts two very intriguing questions: (1) how does the energy flow through the 6D phase space and what therefore is the structure of the fluctuations in this space: their spectra, phase-space correlation functions etc. (these fluctuations are best observed in the solar wind, but these days we can also measure density and magnetic fluctuations in galaxy clusters, via X-ray and radio observations); (2) when turbulent fluctuations are dissipated into particle heat, how is their energy partitioned between various species of particles that populate the plasma: electrons, bulk ions, minority ions, fast non-thermal particles (e.g., cosmic rays).

The latter question is particularly important for extragalactic plasmas because all we can observe is radiation from the particles and knowing where the internal energy of each species came from is key to constructing and verifying theories both of turbulence and of macroscale dynamics and thermodynamics. This project has an analytical and a numerical dimension (which of these will dominate depends on the student’s inclinations).

Analytically, we will work out a theory of phase space cascade at spatial scales between the ion and electron Larmor scales (we have done some preliminary work, so we know how to start off on this calculation, but obviously at some point we’ll be wading into unchartered waters). Numerically, we will simulate this cascade using “gyrokinetic” equations – an approach in which we average over the Larmor motion and calculate the distribution function of “Larmor rings of charge” rather than particles (this reduces the dimension of phase space to 5D, making theory more tractable and numerics more affordable).

Background Reading:
1. A. A. Schekochihin et al., “Astrophysical gyrokinetics: kinetic and fluid turbulent cascades in magnetized weakly collisional plasmas,” Astrophys. J. Suppl. 182, 310 (2009)
2. A. A. Schekochihin et al., “Phase mixing vs. nonlinear advection in drift-kinetic plasma turbulence,” J. Plasma Phys. 82, 905820212 (2016)
3. Y. Kawazura, M. Barnes, and A. A. Schekochihin, “Thermal disequilibration of ions and electrons by collisionless plasma turbulence,” PNAS 116, 771 (2019)
4. R. Meyrand, A. Kanekar, W. Dorland, and A. A. Schekochihin, “Fluidization of collisionless plasma turbulence,” PNAS 116, 1185 (2019)
5. A. A. Schekochihin, Y. Kawazura, and M. A. Barnes, “Constraints on ion vs. electron heating by plasma turbulence at low beta,” J. Plasma Phys. 85, 905850303 (2019)


Interaction between tides and convection in stars and giant planets

Supervisor: Caroline Terquem

A large proportion of stars are found in binary systems.  When the distance between the two stars in such systems is small enough, oscillations are excited in each of the stars by the tidal potential of its companion.  These tidal waves are dissipated in the convective regions of the stars.  Such dissipation of energy leads to circularisation of the orbits.  Observations show that close orbits are circular whereas wider orbits have eccentricities.  The period at which the transition occurs for a type of stars is called the 'circularisation period'.  Until now,  theoretical studies, which have relied on mixing length theory to model convection, have predicted circularisation periods significantly smaller than the observed ones.   However, we have just developed a new description of the interaction between tides and convection that yields the observed values of the circularisation period.  Similarly, this new formalism is able to account for the rate of tidal energy dissipation which is needed in giant planets to explain the orbital evolution of their satellites, and which had been a puzzle for the last 50 years.  There is a large number of problems that should be revisited using this new description, and this is the aim of the project.  These studies can be applied to a variety of systems, including binary systems with two stars, or with one star and a giant planet, or with a giant planet and a satellite.   The project will use analytical and numerical tools.