Computational Astrophysics PhD Projects
Research projects on offer in our Simulations group:
The dynamics of seed black holes in the early Universe
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
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
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.
Precision cosmology with the Lyman-Alpha forest
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).
Building a Better Interstellar Medium for Galaxy Simulations
The study of galaxy formation combines several nonlinear processes at scales both incredibly large (gravity, hydrodynamics, magnetic fields, cosmological expansion) and incredibly small (atomic physics, molecule and dust formation, cosmic ray transport). Because of this complexity, much of our theoretical understanding of the evolution of galaxies is owed to the field of simulations.
And yet, at the heart of this enterprise, simulations depend on fairly crude models to determine when and how much gas should be converted into stars. One of the main challenges for improving these models is computing the chemical and thermal evolution of the interstellar medium (ISM). The ISM is a multi-phase mixture of atomic and molecular gas as well as dust grains of various sized and consistencies. This complexity has historically been too computationally epxensive to follow directly in the simulation. However, advances in both computing hardware and software have brought this problem into the realm of feasibility.
In this PhD project, the student will develop a new ISM model and incorporate it into a sophisticated galaxy simulation framework. This new model will significantly improve the way the simulations form stars and open up vast new areas of study with the data produced. The project will contribute to an international team of researchers studying the formation and evolution of Milky Way-like galaxies with frequent opportunities to meet and collaborate. The student will have access to the IfA's computing cluster for development as well as national supercomputing resources for running large-scale simulations.
Galactic Building Blocks at Cosmic Dawn
The formation of the first generation of stars marked the beginning of the era known as Cosmic Dawn. This occurred in the first few hundred million years after the Big Bang. Theory and observation both suggest that the first stars were likely much more massive than the stars we observe today. This is due to the fact that they formed out of primordial gas created in the Big Bang, which contained only hydrogen and helium, but none of the heavier elements (like carbon, oxygen, iron, etc.) that are crucial to and ubiquitous in the modern Universe. The supernovae that marked the deaths of these massive stars created the first heavy elements, which we call "metals". The dark matter halos in which the stars formed were so small that they could not contain these violent supernova explosions, allowing the newly forged metals to disperse into the proto-intergalactic medium. Eventually, these metals would mix into cold, dense gas to form the next generation of stars. These would be the first able to survive to the present day, and indeed can be found scattered through the outskirts of our own Milky Way, the remnants of galactic pre-history.
In this PhD project, the student will explore the origins of the first metal-enriched stars using cutting-edge high-resolution cosmological simulations. These simulations follow the formation of the first stars, their supernovae, and subsequent dispersal of metals into the large-scale structure as the age of galaxy formation begins. There are numerous questions that can be pursued in this project. What was the influence of the first generation of stars? How were metals first incorporated cosmic structures? How and when did dust grains (crucial to star formation today) initially form? What are the observable signatures of this epoch? The project work can take many directions, including analyzing high-resolution simulation data, running new simulations, and developing codes for improved modeling of chemical enrichment. The student will have access to the IfA's computing cluster and will work in an environment with many researchers with relevant expertise in galaxy formation simulations as well as observations of high-redshift galaxies and local stellar populations.
Under the Galaxy Formation & Evolution projects, see also:
- Probing the Dark Universe with Euclid - Dr Alex Hall and Dr Naomi Robertson
- Machine Learning Galaxy Formation -Prof Sadegh Khochfar