DPhil Projects 2021: Observational and Theoretical Cosmology.

The evolution of galaxies in the early universe with the next generation of telescopes
Dr Rebecca Bowler

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At the cutting-edge of Astronomy research is the study of the formation and evolution of the first galaxies. Through breakthrough observations in the past 30 years it has been possible to identify galaxies from when the universe was less than 500 million years old. These galaxies have unusual properties compared to the local universe, showing low chemical enrichment and dust obscuration, and irregular morphologies. This project aims to exploit the new Vera Rubin Observatory (VRO) and Euclid space-mission to discover and analyse galaxies at very high-redshifts (probing the first few billion years). The goal of the project is to understand when and how the most star-forming galaxies formed in the universe. The student will become an expert in the selection of high-redshift galaxies from multi-band photometry. They will then use the resulting samples to constrain the evolution of the number density of these sources (via the luminosity function). There is considerable flexibility in the direction of the project in later years, and the student would be encouraged to apply for follow-up data (e.g. with JWST, ALMA) as well as exploit archival data where available. At the end of the project the student would be in an excellent position to continue working with data from these next generation facilities.

New physics from black hole mergers
Pedro Ferreira

With the discovery of gravitational waves, we now have a new window
onto the physics of black holes and how they might be used to study
fundamental physics. Of particular interest is the possibility that
their might be signatures of new physics that might arise during a
neutron star or black hole binary merger. In this project we will try
to construct both and a numerical and analytic understanding of how,
for examples, scalar degrees of freedom will be excited during such
cataclysmic events. We will build on my recent work on
"Forecasts for low spin black hole spectroscopy in Horndeski gravity",
O. Tattersall and P.G.Ferreira, Phys. Rev. D99, 104082 (2019)
"Quasinormal modes of black holes in Horndeski gravity",
O.J. Tattersall and P.G. Ferreira, Phys. Rev. D97 104047 (2018)
"Speed of gravitational waves and black hole hair", O.J. Tattersall,
P.G. Ferreira and M. Lagos, Phys. Rev. D97, 084005 (2018)

and on my work on scalar-field accretion around black holes :
"Growth of massive scalar hair around a Schwarzschild black hole", K
Clough, P.G. Ferreira, M. Lagos, Phys. Rev. D100, 063014 (2019)

The impact of magnetic fields on gas accretion onto supermassive black holes and AGN feedback: the next frontier of galaxy formation cosmological simulations
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 feedback, take 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 and Beckmann Slyz & Devriendt 2019) 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 a 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.

The impact of primordial magnetic fields on dwarf galaxies
A. Slyz and J. Devriendt

The smallest galaxies in the Universe, called dwarf galaxies, hold the key to many mysteries regarding the properties of the Universe. They are generally cast as responsible for forcing it to come out of the ‘Dark Ages’, making it transparent to ionising photons a second time, one billion years after the Big Bang, at the end of an epoch called ‘re-ionisation’. They also feature density profiles in tension with those predicted by the standard Lambda Cold Dark Matter model, being cored instead of cuspy at their centres. However, a largely unexplored area is the importance of dwarf galaxies for the study of primordial magnetic fields (PMFs). If they were generated before recombination, PMFs can act on the ionized baryons and drive density perturbations via the Lorentz force which then couple to the dark matter (DM) via gravity. For high values of PMF strengths (still compatible with current observational constraints) , this effect alters the number of objects of a given mass (mass function) in the dwarf galaxy regime. In comparison to the standard one predicted by inflation alone, the PMF modified mass function yields an increased number of dwarfs.

The goal of this DPhil project is to study a statistical sample of dwarf galaxies with different PMF modified initial conditions, explicitly taking into account magnetic fields and cosmic rays. Comparison to local dwarfs will provide constraints on primordial magnetic fields, helping to elucidate which theoretical magneto-genesis models are favoured. 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) will also be generated..

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

Testing Galaxy Evolution models with synthetic and real data sets
Dimitra Rigopoulou, Julien Devriendt & Niranjan Thatte

Cosmological simulations, like the New Horizon suite, can now achieve spatial resolutions of ~50 pc at redshifts z > 1, comparable to that achieved by the latest generation of mm/sub-mm telescopes (ALMA), and soon to be achieved by the ELT+HARMONI at near-infrared wavelengths. This provides a unique opportunity to test predictions of galaxy evolution models by comparing “mock observations” of simulated galaxy properties with ALMA and ELT observations.
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Our method uses cosmological simulations that forward propagate primordial density fluctuations consistent with observations of the cosmic microwave background, creating individual galaxies at high spatial resolution, whose kinematic, morphology and dynamical properties are consistent with observed ensemble properties of the population at the corresponding redshift. As the input physics (e.g. star formation laws) for the simulation is well understood, the resulting objects provide dynamically stable mock galaxies consistent with physical laws and cosmological evolution models at the appropriate redshift. We have developed a method for post-processing these mock galaxies, computing gas emission line intensities using CLOUDY radiative transfer computations in each cell, to get realistic model galaxy observations with self-consistent kinematics and dynamics.

We are looking for a motivated D.Phil student who has a keen interest in state-of-the-art numerical simulations, and is eager to work at comparing simulations with observations to test models of galaxy evolution. The successful candidate will gain expertise in radiative transfer, cosmological simulations, data reduction and analysis. The project work involves working with astronomical data sets, both real and simulated.