Postgraduate opportunities

We are major contributors to XXL, a 50 square-degree X-ray survey, and the largest XMM-Newton science project to date. This survey identifies clusters of galaxies from their X-ray emission and then characterises these systems using a wealth of multi-wavelength data. We use these systems to study the cosmic evolution of both the gaseous intra-cluster medium and of the galaxies that populate these systems. We are interested in how the clusters themselves grow over cosmic time, how the galaxies within them evolve both through time and with the dynamical state of the hosting clusters and how the properties of these systems can be used to constrain our understanding of the Universe as a whole. XXL is enabling the identification of new clusters over a huge range of distance from us, the most distant being seen in light emitted when the Universe was only a quarter its present age. Given this huge time-scale, the survey is ideal for studies of the time-evolution of galaxy clusters.

Possible PhD projects based on XXL include: the search for the most distant clusters; studying galaxy evolution in clusters; estimating masses of clusters from X-ray data to contrain cosmological models; studying the scaling properties of the clusters and their evolution; observing the Sunyaev-Zel'dovich effect of the clusters. This work will be carried out in collaboration with astronomers around the globe. In certain cases, these collaborations make it possible for PhD research projects to be carried out both in Bristol and in partner institutions. Currently, there is an opportunity to carry out PhD work on the evolution of cluster galaxies jointly between Bristol and the AAO and Macquarie University in Australia. Group members involved in this programme are Mark Birkinshaw, Malcolm Bremer and Ben Maughan.

A PhD project is being offered in the area of planetary astrophysics on understanding the formation and evolution of the outer solar system. Several fundamental questions about the outer solar system remain unanswered, including: What was the initial size distribution of planetesimals, the building blocks of planets? What was the role of binaries during accretion into larger bodies? What processes led to the observed diversity of dwarf planets? What was the balance between collisional erosion and dynamical clearing in removing mass from the Kuiper Belt?

Dr. Leinhardt currently has funding for a joint studentship with Prof. Stewart-Mukhophadhyay (Harvard University) to investigate the physics of collisions between icy bodies in the outer solar system using state of the art numerical simulations. The results from these experiments will be used to develop analytic models to describe collision outcomes for use in planet formation and evolution studies and to investigate the role of collisions in shaping the diversity of dwarf planets in the Kuiper Belt.

A successful candidate will be expected to spend half of their time at the University of Bristol and half of their time at Harvard University in the USA. The PhD would be awarded by the University of Bristol. Group member involved in this programme: Zoe Leinhardt.

The scattering of the microwave background radiation by gas in clusters of galaxies is used to study cluster atmospheres and as a tool to measure the parameters that define the large-scale structure of the Universe. Observations are made using ground-based telescopes in the radio and sub-mm, and compared with X-ray data from satellite observatories. Theoretical aspects of the scattering process are also studied. Group member involved in this programme: Mark Birkinshaw.

The investigation of the numbers and properties of faint, distant galaxies can give clues to the processes involved in the formation and evolution of galaxies and as to the overall nature of the Universe itself. Our observational work uses large ground based telescopes as well as data from the Hubble Space Telescope to study the high redshift universe. Group member involved in this programme: Malcolm Bremer.

At the centre of every galaxy is a super-massive black hole, and in a small fraction of galaxies, called Active Galactic Nuclei (AGN), a significant quantity of gas is flowing into the black hole. This accreting gas forms a disc which emits X-rays that we observe with space-based observatories such as Chandra, XMM-Newton and Suzaku. Detailed modelling of the X-ray spectrum from AGN can tell us about the black hole (e.g., is it spinning?) and the geometry of the X-ray source and disc. There is an opportunity for both theoretical and observational X-ray projects related to black hole accretion flows. In particular, I am interested in looking at the variability characteristics of AGN (e.g., time lags as a function of X-ray energy and Fourier frequency) and how this can be used to "echo map" the inner disc and infer the physics of the accretion flow. Group member involved in this programme: Andrew Young.

We make use of both satellite and terrestrial observatories to study the physics of active galaxies. The X-ray and radio properties form a particular focus of our work, in which we study the inner geometry and radiation mechanisms as well as the effects of hot gaseous environments on radio-source propagation and evolution. We expect further discoveries from our observing time on the X-ray observatories Chandra and XMM. Group member involved in this programme: Diana Worrall.

X-ray observations of galaxy clusters provide a powerful way to study their properties. We have several programmes underway to study these massive galaxy structures and better understand them. A key goal is to improve our ability to measure the masses of clusters of galaxies - an observational challenge as the systems are dominated by dark matter. Using our improved mass measurements, we aim to test the predictions of different cosmological models against observations of samples of galaxy clusters. Group member involved in this programme: Ben Maughan.

Simple models predict that galaxy clusters should have fractal-like behaviour, with low mass clusters being identical to scaled down versions of high mass clusters. However, observations show that this is not the case. We are studying the scaling properties of clusters to learn about extreme physical processes related to cluster mergers, energy injection from active galaxies, and runaway cooling processes, that are believed to be responsible for breaking expected self-similar scaling. Group member involved in this programme: Ben Maughan.

Galaxy clusters act as gravitational lenses, distorting and magnifying the images of background sources. Radio, sub-millimetre and infra-red observations are an efficient way to find new lenses, the study of which will enable us to estimate the mass of the lensing clusters. This provides en excellent way to examine the density structure and masses of these clusters, with a view to understanding the distribution of dark matter and the formation and growth of clusters of galaxies. Group members involved in this programme are Mark Birkinshaw and Ben Maughan.

We have a long term observational programme to obtain spectra for complete samples of objects in certain areas of the sky, regardless of their apparent image morphologies. This allows us to investigate, for example, compact dwarf galaxies with no bias due to pre-selection of `likely looking' targets. Group member involved in this programme: Steven Phillipps.

The majority of galaxies in clusters are, in fact, small and of low surface brightness. Our programme aims to elucidate the true extent, and properties, of this elusive class of object. Group member involved in this programme: Steven Phillipps.

Compact groups of galaxies offer unique laboratories to study galaxy-galaxy interaction, due to their low velocity dispersion compared to the larger galaxy clusters. In this project we will use the new catalogue (McConnachie et al. 2009) of compact galaxy groups from the Sloan Digital Sky Survey (SDSS - Data Release 6) to explore the impact of such interactions on galaxy morphology, star formation etc. This will involve studies of both the imaging and spectra available in SDSS for a large number of the ~3000 compact groups in the catalogue. The student will gain experience in photometry, analysis of spectra, and the use of large-scale astronomical databases, which will prove useful in their future studies.

The HST Treasury Survey of the Coma Cluster has obtained deep, high resolution images of large areas of the Coma Cluster, the closest very massive galaxy cluster. We have several projects concerned, generally, with the dwarf galaxy content of the cluster and evolutionary processes which affect the dwarfs in this dense environment. In addition we have a specific project to explore what are called post-starburst galaxies and the populations of star clusters which may form in these bursts, thought often to be induced by close interactions and mergers.

This is a theoretical and numerical project to compute the time-dependent X-ray spectrum produced by X-ray flares above an accretion disk. The project requires integrating photon paths through the Kerr metric from point-source flares at arbitrary locations above the accretion disk to calculate the flux incident on the disk, and from the observer to the accretion disk to determine what will be observed. The purpose of this project is to produce a general purpose model that can be incorporated into computer programs used to fit X-ray observations of black hole accretion disks. The project is theoretical and numerical, and a knowledge of general relativity (e.g., the Kerr metric) is required.

The bipolar jets produced by accreting compact objects can generate intense X-ray radiation. These X-rays will illuminate the accretion disc, and produce an iron fluorescence line. Previous calculations of this phenomenon have assumed that the accretion disc rotation axis and jet axis are aligned. If these are misaligned, it is possible that the X-ray illumination of the disc is enhanced and hence the iron fluorescence line is also enhanced. The objective of this theoretical and computational / numerical project is to calculate the iron line produced by accretion discs illuminated by a misaligned jet.

This project would be related to the observations we're currently planning with the new array receiver on the Torun 32-m telescope in Poland. With this system we are planning to make some maps of the structures of extended radio sources (which are fairly bright), and of the Sunyaev-Zel'dovich (SZ) effects of distant clusters of galaxies (which are very faint). The SZ effect data can be used to find where hot baryons are located in the Universe, but to be effective we have to be able to construct extremely sensitive images from the mapping data. This is difficult since we are observing through a time-variable atmosphere, and the purpose of the project is to investigate a number of possible mapping techniques to decide which will be best, and then to use it to make some maps. The project will involve learning about how radio telescopes work, and doing some coding to calculate the effectiveness of the mapping methods and reduce the data when they come in.

As galaxies move through the atmospheres in clusters of galaxies they must emit long-wavelength magnetoacoustic waves. These can propagate through the cluster atmosphere, and will dissipate to cause heating. The question of cluster heating is of considerable importance: without some heat source, perhaps this one, cluster atmospheres would undergo collapses within a Hubble time, generating hypermassive galaxies. This project will be theoretical (and likely computational), and should produce an interesting short paper within the year.

Clusters of Galaxies are highly luminous X-ray sources, allowing them to be studied observationally out to high redshifts. Such studies have given conflicting results on the evolution of galaxy cluster properties, and there is debate in the literature as to whether clusters obey the simplest model for their evolution or if additional factors are present. In this project we will investigate the evolution of the X-ray properties of galaxy clusters using several large samples at different redshifts to properly include selection biases in our analysis. The project will consist of analysis of large amounts of observational data, and statistical treatment of the results, including sample selection biases.

Simple models predict that galaxy clusters should have fractal-like behaviour, with low mass clusters being identical to scaled down versions of high mass clusters. However, observations show that this is not the case, and give evidence for extreme physical processes at work in clusters (such as mergers, active galaxy energy input, and runaway cooling) that break this self-similar scaling. This project will be an observational study of low mass clusters, where the similarity breaking is found to be the strongest, in order to better understand the differences from fractal behaviour, and their causes.