Potential PhD projects: Extragalactic Astrophysics and Cosmology

Project: Cosmology from gravitational lensing observations of galaxy clusters

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).

Testing gravitational lensing measurements of galaxy clusters

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).

Project: The evolution of galaxy groups and clusters - astrophysics and cosmology

Supervisors: Prof. Trevor Ponman and Dr Graham P. Smith

The distribution of cluster masses and their evolution is one of the key probes of cosmology. However, most of the mass is dark and hence has to be inferred from the study of the visible mass in clusters, except in cases where gravitational lensing can be used (mostly for rather massive clusters at intermediate redshifts). However, this visible mass (the galaxies and hot gas within clusters) is sensitive to the effects of "cosmic feedback": the processes whereby gas is heated by the energetic input from supernovae and accreting supermassive black holes. This is especially true in lower mass clusters - galaxy groups.

These feedback processes are of great interest in themselves, and are a major factor in the development of cosmic structure. Without feedback, most baryonic matter would now be locked up in stars, whereas observations show that 85% is not. Given the coupling between feedback and cluster structure, it is important to study cluster cosmology and the astrophysics involved in cluster evolution together.

There is another problem. The clusters we detect in astronomical surveys are not an unbiased sample of what is there, but will be influenced by the nature of the survey. For example, X-ray surveys detect clusters via the emission for their hot gas, and this approach will tend to preferentially pick out clusters containing dense, hot gas. To allow for such observational selection effects one needs to have a good model for the properties of the cluster population (otherwise one has no way to assess what one might be missing).

To address these problems requires the bringing together of well-constructed observational surveys and high quality cosmological simulations. The simulations help us to correct for observational biases, whilst the observations provide a check on whether the simulations are realistic. Our programme to do this uses data from the large XXL X-ray survey and the deep GAMA galaxy redshift survey, in conjunction with high quality cosmological simulations conducted by our collaborator Ian McCarthy at Liverpool.

This project would provide an opportunity to work with both observational and simulated data (both XXL and GAMA are highly multi-wavelength) to study the astrophysics and evolution of groups and clusters, and explore the implications for cosmology.

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

Project: Pan-chromatic Study of Galaxies and their Environment

Supervisors: Dr Graham P. Smith and Dr Felicia Ziparo

According to the hierarchical growth of structure formation, galaxies experience a wide range of environments throughout their lives, from the low-density "field" regions to dense regions in the cores of galaxy groups and clusters. It is well known that the properties of galaxies are related to their environment, suggesting that environment plays a role in galaxy evolution. However the physical processes responsible for transforming gas rich star-forming galaxies into gas poor early type galaxies, and the role of these processes as a function of environment remains a controversial and hotly debated topic. Dr Smith's group, and collaborators in Arizona, Japan, Finland, and Chile, have assembled a superb dataset with which to tackle this problem, as part of the Local Cluster Substructure Survey. The data include ultraviolet, optical, and infrared imaging with GALEX, Subaru, and ESA's Herschel Space Observatory, plus optical spectroscopy from ESO's VLT. An opportunity exists for a student to join the group to use these data to investigate galaxy evolution in field, group, and cluster environments. Example projects include (1) integral field spectroscopy of galaxies in groups falling into clusters, and (2) comparing optical, ultraviolet, and infrared indicators of star formation.

For more information please contact Dr. Graham Smith (gps[at]star.sr.bham.ac.uk) or Dr. Felicia Ziparo (fziparo[at]star.sr.bham.ac.uk).