Supervisor: Dr Graham P. Smith

The mass, shape, internal structure, and matter content (both luminous matter and dark matter) of galaxy clusters are all sensitive to the details of our cosmological model. This is due, in large part, to the privileged position of galaxy clusters at the top of the "cosmic food chain" - i.e. they are the most massive objects in the universe, in which massive structures such as galaxy clusters grow hierarchically by ingesting smaller systems. Despite major progress in recent years on measurements of cosmological parameters, there remain many exciting open cosmological questions, relating to (for example) the existence/properties of "dark matter" and "dark energy", the validity of General Relativity on cosmological scales, and the spectrum of initial density fluctuations in the early universe. As the Principal Investigator on the Local Cluster Substructure Survey (LoCuSS), Dr Smith has assembled an unprecedented wealth of data with which to probe the mass and internal structure of galaxy clusters, including sensitive gravitational lensing observations from the Subaru Observatory and Hubble Space Telescope, and complementary data at infrared, X-ray, and millimetre wavelengths. An opportunity therefore exists for a student with cosmological interests to work with Dr Smith on the cosmological exploitation of the LoCuSS dataset, exploring the topics outlined above and/or new ideas that emerge during the timespan of the PhD.

For more information please look up Dr Graham P Smith's webpage
or e-mail him (gps[at]star.sr.bham.ac.uk).

Supervisor: Dr Graham P. Smith

The astrophysics and cosmology community is investing heavily in several surveys that aim to measure the dark energy equation of state parameter (w=rho/P) using galaxy clusters, to per cent level precision. These surveys represent one of four independent probes, the others being type Ia supernovae (SNIa), baryon acoustic oscillations (BAO), and cosmic shear. The success of cluster-based dark energy experiments, just like the other methods, will depend on the precision to which systematic errors can be identified, controlled, and (preferably) eliminated. In a nutshell, cluster cosmology depends on being able to measure the mass of galaxy clusters reliably. In principle the tool of choice for measuring galaxy cluster mass is gravitational lensing (the deflection of light by mass), however the reliability of lensing- based mass measurements has yet to be tested to the levels required by dark energy experiments. Dr Smith's group are playing a leading role in the global effort to test the reliability of lensing-based measurements of cluster mass, via his leadership of both the Local Cluster Substructure Survey (LoCuSS), and the mass measurement working group of the upcoming Ultimate XMM Extragalactic Survey (XXL). An opportunity exists for a student to join Dr Smith's team to work on stringent tests of lensing-based mass measurement methods using both real observational data from Subaru Observatory, and also from synthetic observations of simulated clusters. The results from this thesis will contribute directly to the success of LoCuSS, and XXL, and will also help to define the state of the art methods upon which other cosmological surveys will depend.

For more information please look up Dr Graham P Smith's webpage
or e-mail him (gps[at]star.sr.bham.ac.uk).

Supervisors: Prof. Trevor Ponman and Dr Somak Raychaudhury

Galaxy groups are very important cosmic structures for two reasons: (i) they account for around 50% of the galaxies in the local Universe, and are therefore the most important galaxy environment, and (ii) they are especially sensitive to the poorly understood processes known as "feedback", whereby matter is prevented from cooling catastrophically by energy pumped into surrounding gas by supernova explosions or active galactic nuclei. The main constituents of galaxy groups are the galaxies themselves, the hot intergalactic gas which emits X-rays, and the dark matter which typically dominates their mass, as it does in richer galaxy clusters.

Our group is one of the foremost in the world in the study of galaxy groups, and we have a number of international projects underway to study their structure and evolution, and to compare their properties with the results of cosmological simulations. One of our main aims is to improve the understanding of cosmic feedback processes, which is the most serious factor limiting our ability to simulate the evolution of structure in the Universe. New postgrads could get involved in one or more of these projects, depending on their strengths and interests:

(a) The XI project - In a collaboration with the Carnegie Observatories in Pasadena, we launched the first systematic and detailed study of the X-ray and optical properties of a proper statistical sample of groups, drawn from the 2dF galaxy redshift survey. A sample of 25 groups is being targetted with the IMACS multi-object spectrograph on the 6.5m Baade/Magellan telescope at Las Campanas, and the XMM-Newton X-ray observatory, the most sensitive X-ray telescope ever flown. The resulting datasets allow detailed study of galaxy membership and dynamics, and of the properties of the hot intergalactic medium in groups. Initial results from this project were reported here, but we now full Spitzer far-IR coverage of these groups to study galaxy activity within them.

(b) We have recently initiated two new projects to study galaxy groups. The CLoGS project aims to survey the groups in the Local Universe (out to 80 Mpc), where measurements are at their most sensitive, and a wealth of multi-wavelength data is already available. We have already been awarded observations with the Chandra and XMM-Newton X-ray observatories, in collaboration with scientists at Harvard, to establish the properties of hot gas within these groups, and have a programme of low frequency radio observations to study active galaxies within these groups. Secondly, we are leading the study of the X-ray properties of galaxy groups drawn from the GAMA deep galaxy redshift survey. This enables us to select groups reliably from their optical properties (using simulated groups to check our procedures), and then to study the hot gas they contain, and its relationship to their galaxy properties.

(c) In our Chandra Deep Group Survey, we are conducting a survey of the X-ray and multi-wavelength properties of groups at the highest possible redshifts, using the deepest available Chandra observations, complemented by our own optical observations with Gemini, in order to study the evolution in group properties. This evolution provides a key test for feedback models.

(d) All the above observational studies benefit greatly from comparison with the results of cosmological simulations. With the recent arrival in our group of Ian McCarthy, we are now fully involved in the OWLS simulation programme. This suite of comsological simulations is specifically designed to explore cosmic feedback from active galaxies and supernovae. The first two papers comparing these simulations with observed group properties can be found here and here. Fruitful interplay between observations and simulations is now a major feature of our programme.

The above projects (and others) offer possibilities for X-ray, optical and radio observations and analysis together with the chance to work with data from cosmological simulations, to advance our understanding of the evolution of both galaxies and groups.

For more information please contact Prof. Trevor Ponman (tjp[at]star.sr.bham.ac.uk).