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Computational Astrophysics PhD Projects

Research projects on offer in our Simulations group:

The dynamics of seed black holes in the early Universe

Dr Ricarda Beckmann

Almost every massive galaxy in the local Universe contains a massive black hole in the centre, and there is growing evidence that they might exist in dwarf galaxies as well. There is no known way to make a supermassive black hole directly. Instead, present day black holes grew from smaller black holes, so called seed black holes, over billions of years. As they did so, their feedback energy shaped the gas content and star formation history of their host galaxy. While massive black holes are central to our current model of galaxy evolution, we as of yet have a very poor understanding of how this coevolution between black holes and galaxies began. It was long thought to be trivial that young galaxies would acquire a seed black hole early on in their evolution, which then grew into the massive black holes we observe today. Recent work has shown that the dynamical evolution of black holes in and around early galaxies is complicated, and that it is difficult for early black holes to settle into galaxies and begin their coevolution.

In this PhD project, the student will explore how small, newly formed black move in and around galaxies to understand when and how they settled into the kind of galaxies where we see massive black holes today. To do so, we will build a state-of-the-art numerical simulations of the early Univese specifically designed to study black hole dynamics. The simulations will follow the formation and evolution of early galaxies, and the formation and orbital evolution of black holes in and around them. Using the simulations, the student will study where and how black holes are born and how we can combine simulations with observations of the early Universe from observatories such as the James Webb Space Telescope (JWST) and the gravitational wave observatory LISA to  provide testable predictions. As part of the project, the student will learn to run their own cosmological simulations on compute clusters and analyse them. Python and or coding knowledge is a plus for the project.

The Origin and Fate of High-Redshift Dusty Star-Forming Galaxies

Prof Romeel Davé

JWST is revealing that the population of massive galaxies at high redshift provide many challenges to models of galaxy formation. These include their star formation efficiency, the number of quenched systems, and their high dust and metal contents. In the canonical scenario, massive galaxies grow quickly within the densest regions of the cosmos, form stars that enriches their gas and creates dust, and eventually grows a black hole that drives their quenching. But does this scenario actually work? Currently, the evidence for this is circumstantial, and the various multi-wavelength observations are difficult to compile into a coherent story.

In this project, the PhD student will examine the role of the dusty star-forming galaxies (DSFGs) at high redshift in the evolution of the massive galaxy population. The primary tool will be a new suite of state-of-the-art numerical simulations called Kiara, which is the successor to the Simba simulations that are fairly unique in reproducing observations of DSFGs. Kiara improves upon Simba with new modules featuring better dust and star formation physics, pushing the frontier in interstellar medium modelling within a cosmological context. The student will begin by analysing galaxies in Kiara, but eventually the goal is to run their own suite of DSFG zoom simulations to probe the detailed physics at substantially higher resolution using the Kiara code. The goal is to explore evolutionary histories, physical conditions, and environmental factors that give rise to DSFGs, how they subsequently transition into quenched galaxies, and how they are impacted by processes such as mergers, accretion, and feedback. The student will also conduct detailed comparisons to the latest data on DSFGs from JWST, ALMA, and other telescopes, in order to carefully test the model and understand how the evolutionary stages of DSFG are manifested in observations.

The student is expected to come in with a good understanding of galaxy formation and evolution and strong python code development skills; knowledge of C/C++ is a plus. They will join the international Kiara team led out of Edinburgh, and serve as the liaison to several major observational efforts that we are a member of, such as PRIMER and NGDEEP on JWST. This is a unique opportunity for a student to grow into a leadership role in a high-profile and rapidly-moving area of modern astrophysics.

The Baryon Cycle During Cosmic Morning

Prof Romeel Davé

Galaxies live in an evolving balance between galactic inflows, self-regulating outflows, and star formation, collectively known as the baryon cycle. Today, the baryon cycle is believed to govern the properties of galaxies, but it remains challenging to observe because many of the processes are occurring within diffuse circum-galactic medium (CGM). A unique probe of this is absorption line spectroscopy, which can probe the physical, dynamical, and chemical state of the CGM, albeit along 1-D skewers. So far this has only been possible in rare and sparse cases, but all that is about to change with the advent of the Extremely Large Telescope (ELT). With its large leap in collecting area, ELT will enable CGM spectroscopy using individual background galaxies at redshifts 2 and beyond, increasing CGM sampling by orders of magnitude. But to assemble this into a coherent story of how the baryon cycle governs galaxy evolution requires the use of cosmologically-situated simulations that can accurately track all the relevant physics.

In this project, the PhD student will use our new forefront Kiara simulations to explore CGM absorption from Cosmic Noon (z~2) back to Cosmic Dawn (z~6). Kiara, the successor to the Simba simulations, includes new modules specifically designed to improve chemical enrichment and the realism of how metal-bearing outflows interact with CGM gas. By generating mock HI and metal absorption lines in Kiara, the student will investigate the connection between absorption features and the dynamical and physical state of the CGM, and how they trace inflowing and outflowing gas. This will involve developing approximate radiative transfer tools, guided by our complementary radiative hydrodynamic simulations Kiara-RT, that will be essential to properly model the ionisation state of the CGM. The student will connect CGM properties to galaxy properties, illustrating the role that the CGM plays in galaxy growth and transformation, and understanding how environmental processes and mergers impact the baryon cycle. Finally, the student will make forecasts for galaxy-galaxy absorption using ELT integral field spectroscopy with HARMONI (being built here at the Royal Observatory), in order to motivate and guide this emerging field.

The student is expected to come in with a good understanding of galaxy formation and evolution and strong python code development skills; knowledge of C/C++ is a plus. They will join the international Kiara project led out of Edinburgh, and eventually be in charge of the CGM absorption line program at high redshift within the Kiara team. This is a unique opportunity for a student to grow into a leadership role in a high-profile and rapidly-moving area of modern astrophysics.

Primordial Black Holes as Dark Matter

Prof Sadegh Khochfar

The existence of primordial black holes is a natural prediction of cosmological scenarios. Dark matter, while proposed as a fundamental part of structure formation in the Universe, has not been directly detected so far. It has been suggested that a fraction of dark matter in the Universe could be composed of primordial black hotels that formed  in the early Universe. If this proves true, it could naturally explain the current lack of dark matter detection in particle accelerators.

In this project we aim to study the impact on structure formation from primordial black holes. As part of the project the student will run the highest resolution state-of-the-art numerical simulations of self-consistently seeded primordial black holes and investigate how the assembly of dark matter haloes changes as we change the fraction of dark matter being in the form of primordial black holes. We will investigate the impact of Hawking Radiation emitted from these black holes and how it influences the baryonic physics in its vicinity.

Precision cosmology with the Lyman-Alpha forest

Prof Avery Meiksin

Intergalactic space is filled with a pervasive medium of ionized gas, the Intergalactic Medium (IGM). Detections of the IGM in the spectra of high redshift quasars reveal a highly fluctuating medium: the Lyman-Alpha forest. The statistics of the fluctuations are well-reproduced by numerical simulations involving dark matter and baryons. As such, the IGM offers an opportunity to probe the primordial density fluctuations on scales unavailable to other methods.

This project will involve the student in numerical cosmological simulations of the IGM to investigate the expected shape of the Lyman-Alpha forest flux power spectrum and its relation to the dark matter primordial power spectrum. The student will be a member of a group involved in cosmological structure simulations at the Edinburgh Centre for Computational Astrophysics (http://www.roe.ac.uk/~aam/ecca).

Instabilities in rotating stellar systems

Prof Steve Tobias, Dr Anna Lisa Varri

The analysis of the stability properties of self-gravitating, uniformly rotating equilibria is a classical fluid dynamics problem with a distinguished history, starting with the study of the ellipsoidal figures of equilibrium (Chandrasekhar 1969). In the theory of rotating stars (Tassoul 1978), the investigation of the properties of self-gravitating rotating fluid bodies has been generalized to the case of configurations with non-uniform density, especially polytropic fluids with solid-body (James 1964) and differential rotation (Stoeckly 1965). Yet the important problem of the stability of spheroidal rotating kinetic stellar systems has rarely been explored (Rozier et al. 2019), and the connection with the corresponding fluid systems is only partly understood (Vandervoort 1980).

Stimulated by the advent of gravitational wave astrophysics, there has been a booming interest in the study of the stability of differentially rotating self-gravitating fluids, kindled by the discovery (Centrella et al. 2001) of an unstable m = 1 mode in polytropes with strong differential rotation and surprisingly low values of the ratio of the rotational kinetic energy to the gravitational energy (T/W). The study of the stability of differentially rotating spherical shells (Watts et al. 2005) suggests that the unstable modes within low-T/W differentially rotating configurations are characterised by corotation within the system and that the degree of shear must be sufficiently high for the instability to occur. But a comprehensive understanding of this phenomenon is still lacking.

This PhD project proposes to investigate the emergence and saturation of dynamical instabilities in spherical and axisymmetric differentially rotating equilibria, with emphasis on such ‘low-T/W’ class of instabilities, by means of perturbative and numerical techniques. We will first address the spherically symmetric fluid problem in a Newtonian setting and subsequently evaluate (i) the impact of general relativistic effects, starting from a post-Newtonian approximation, (ii) the role of magnetic field, starting from a magnetised, stably stratified configuration (Dymott et al. 2024). On the technical side, the student will gain experience in several modern numerical modelling techniques, including solving elliptic partial differential equations with spectral methods, by means of state-of-the-art tools (e.g., Burns et al. 2020). Longer-term developments of the project include the ambition to transfer to the kinetic setting any knowledge established on the fluid dynamical one.

Such an investigation has the potential to enrich our fundamental understanding of the role of angular momentum in the dynamics of both fluid and kinetic stellar systems, with significant implications on multiple areas of astrophysical interest - from rotating massive stars and the resulting stellar-mass black holes, to compact objects and their gravitational wave emissions, as well as differentially rotating stellar systems, such as galactic nuclei and stellar clusters.

Under the Cosmology projects, see also:

Under the Exoplanet projects, see also:

Under the Galaxy Formation & Evolution projects, see also:

 

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