School of Physics & Astronomy

Find a PhD Project Here

Opportunities for fully funded PhD or EngDoc research projects are available in all fields of research within the School. You may search for current projects on this page. APPLY HERE for a PhD Place.

 PhD in Photonics
 PhD in Condensed Matter
 PhD in Astrophysics

Search current PhD opportunities in the School of Physics & Astronomy:-




Astrophysics

Determining the origins of galaxy bimodality using hierarchical Bayes methods
Wild, Dr Vivienne - vw8@st-andrews.ac.uk

How galaxies form and evolve is one of the outstanding questions of modern astrophysics. We have known for many decades that massive galaxies come in two main types - elliptical/quiescent and spiral/star-forming. However, it remains largely unknown why some galaxies are still forming stars while others are "red and dead". Extremely large galaxy surveys are providing an increasingly detailed census of both local and distant galaxies. Considerable progress is being made on quantifying the changing demographics of the galaxy population over the majority of the age of the Universe, but significant improvements in methods are required to dramatically improve our understanding of the physics behind the observable properties of galaxies.

In the last decade a Bayesian approach to the fitting of sophisticated models to high quality spectra and/or multiwavelength photometry has become common place in the analysis of galaxy spectral energy distributions (SEDs) at all redshifts (Walcher et al. 2011). The result is robust physical properties, such as galaxy stellar mass, dust content and star formation history, together with well quantified degeneracies between these parameters. However, by treating galaxies as independent entities to determine their physical properties, we are missing vital population information. A better approach would be to treat galaxies as a population of objects with a common origin and common underlying variables. This could tighten constraints on the physical properties of individual galaxies as well as the underlying relationships that impact the life of galaxies. It could also allow us to extract information from larger surveys with lower quality observations.

Hierarchical Bayes techniques have been used in the astronomical literature to solve problems as diverse as quasar redshift estimation (Bovy et al. 2011), exo-planet orbit analysis (Hogg et al. 2011), properties of supernovae light curves (Mandel et al. 2009), and photometric redshifts (Leistedt et al. 2016). They differ from standard Bayesian methods by fitting the entire dataset in a coherent manner, instead of single objects as entirely independent entities. By applying these methods to galaxy evolution studies, we will improve our ability to break degeneracies between physical parameters and understand the underlying processes governing galaxy evolution. These methods could be applied to e.g. complete populations of galaxies in spectroscopic or photometric surveys, or entire integral field datacubes of single galaxies.

This interdisciplanary project will be jointly supervised by Drs Vivienne Wild and Michail Papathomas in the Schools of Physics and Astronomy and Mathematics and Statistics respectively. Dr Wild has built her career around developing and applying novel statistical techniques to explore datasets on galaxy evolution, focussing most recently on understanding the nature of post-starburst galaxies. Dr Papathomas is an expert in Bayesian modelling, both in the development of new methods and their application to a wide variety of datasets.

Large extragalactic datasets are already available for analysis, both at low and high redshift. The School of Physics and Astronomy is a member of the UK participation group in SDSS-IV, the fourth generation of Sloan Digital Sky Surveys, a large international collaboration encompassing several astronomical surveys. The methods developed during the project could also be applied to upcoming datasets to which the group has proprietary access such as DESI bright galaxy survey, LSST science verification data and VLT/MOONS near-infrared spectroscopic survey.

The project involves the development of statistical techniques to make them applicable to astronomical datasets. This project would suit students with a background in (astro)physics but strong aptitude for maths and statistics, and students with a background in maths or statistics and interest in astrophysics.

For more information please contact Dr Vivienne Wild and Dr Michail Papathomas (vw8@st-andrews.ac.uk, M.Papathomas@st-andrews.ac.uk).


References:
Bovy J., Myers A. D., Hennawi J. F. et al. arXiv:1105.3975
Hogg D. W., Myers A. D., Bovy J., 2010, ApJ, 725, 2166
Leistedt B., Mortlock D., Peiris H., 2016, MNRAS, 460, 4258
Mandel K. S.; Wood-Vasey W. M., Friedman A. S., Kirshner R. P., 2009, ApJ, 704, 629
Walcher C. J., Groves B., Budavri T., Dale D., Ap&SS, 2011, 331, 1

Surveys:
SDSS-IV www.sdss.org
LSST www.lsst.org
DESI www.desi.lbl.gov
MOONS https://vltmoons.org/science-2/

Dissecting galaxies in transition
Wild, Dr Vivienne - vw8@st-andrews.ac.uk

The number density of `red and dead' elliptical galaxies increases with cosmic time, meaning galaxies must be transitioning from star-forming disks. In this project we will use the SDSS-IV MaNGA integral field survey alongside mock observations of hydrodynamic simulations to move beyond demographics and pin down which physical processes are responsible for this transition, as a function of stellar mass and environment.

Massive galaxies in the nearby Universe typically fall into two distinct populations (e.g. Strateva et al. 2001; Baldry et al. 2004): actively star-forming disk galaxies, and `red and dead' elliptical galaxies with little or no signs of ongoing star formation. The bimodality in the star-forming properties of massive galaxies has existed since at least z~2 (Tomczak et al. 2014), and the strong correlation between star-forming properties and morphology holds to a similar epoch.

The increasing number density of red-sequence galaxies with cosmic time at fixed stellar mass (e.g. Moutard et al. 2016) tells us that galaxies move from the blue-cloud to the red-sequence by having their star formation halted (`quenched'). Coincidentally, the typical star formation rate of galaxies decreases as they exhaust their gas supplies, but this process is unable to account for the alteration in the galaxies' morphologies. Many different mechanisms have been proposed to partly or wholly explain the build-up of the quenched elliptical population with time, such as gas stripping, merger induced starbursts, AGN feedback and morphological quenching.

Catching galaxies that are in the act of transition, and studying both their evolving demographics and their properties in detail, alongside mock observations from state-of-the-art simulations, are the ways to make further progress (Wild et al. 2009). These objects are rare, and only recently have surveys been large enough for us to be able to constrain the number density evolution of green-valley and post-starburst galaxies, alongside the quiescent galaxies we expect them to evolve into.

The advent of highly multiplexed integral field surveys, alongside well developed hydrodynamic galaxy simulations, means the time is perfect for a fully encompassing study of all types of candidate-transition galaxies. The fossil record of a galaxy's formation history is encoded in its morphology, stellar kinematics, stellar populations and dust, ionised gas content and kinematics. The MaNGA survey provides access to all of these, alongside robust control samples, volume correctable number densities, a wide range of environments and stellar masses. Importantly, comparison to large numbers of hydrodynamic simulations is now possible, converted into mock data cubes to be analysed in exactly the same way as the data.

In this project we will perform an integrated analysis of the morphologies, shapes, spatially resolved kinematics and stellar populations of the largest ever sample of local post-starburst, blue-ellipticals, red-spirals and green-valley galaxies observed with an integral field unit, alongside large sets of control samples and hydrodynamic simulations. We will aim to understand what are their ancestors and descendants and thereby understand whether they are truly transitioning populations, and on what timescales. We will use these results to interpret data from high-redshift surveys, where post-starburst galaxies are far more common and potentially important for building the present-day galaxy bimodality (Wild et al. 2016).

This project will be co-supervised with Dr Anne-Marie Weijmans, lead observer for MaNGA.

Strateva et al. 2001, AJ 122, 1861
Baldry et al. 2004, ApJ 600, 681
Tomczak et al. 2014, ApJ, 783, 85
Moutard et al. 2016, A&A, 590, 103
Wild et al. 2009, MNRAS, 395, 144
Wild et al. 2016, MNRAS, 463, 832
MaNGA survey : http://www.sdss.org/surveys/manga/
A complete census of rapidly quenching galaxies over cosmic time
Wild, Dr Vivienne - vw8@st-andrews.ac.uk

It has been known since the 1920's that most local galaxies can be morphologically classified as spirals or ellipticals. In the last century advancing galaxy surveys have demonstrated that morphology strongly correlates with many other galaxy physical properties: spiral galaxies are typically forming stars, contain cold gas and dust, live in less dense environments, and have modest-sized central black holes, whereas elliptical galaxies are “quenched” (not forming stars), surrounded by hot gas, live in denser environments, and have larger black holes. Despite precise statistical characterisation of these properties throughout much of cosmic history, the underlying physics that separates galaxies into star forming vs. quenched, with all attendant correlations, remains unclear. Understanding the *pathways* by which galaxies quench is a crucial outstanding question at the heart of modern galaxy formation and evolution, impacting a range of astrophysical studies from star formation and interstellar medium to dark matter and dark energy.

In this project we will combine the analysis of optical/NIR photometry and spectroscopy to identify and characterise the properties of rapidly quenched galaxies from redshifts of 0 to 2 in the real and simulated Universes. Then we will study their environment and gas properties to identify trends that should be reproducible in advanced cosmological hydrodynamic simulations. Comparison of data and simulations will elucidate the role of rapid quenching in the build up of the red sequence.

This project will work closely with Romeel Davé in Edinburgh (rad@roe.ac.uk), who runs the SIMBA and MUFASA galaxy evolution simulations. Real and mock data is in hand to commence the project. During the lifetime of the project new data will become available from the MIGHTEE survey (HI and radio SFRs) and VLT/MOONS (rest-frame optical spectroscopy of redshift 1-2 galaxies).

References
========

MIGHTEE: https://arxiv.org/abs/1709.01901
MOONS: https://www.eso.org/sci/facilities/develop/instruments/MOONS.html

http://adsabs.harvard.edu/abs/2018MNRAS.473.1168R
http://adsabs.harvard.edu/abs/2016MNRAS.463..832W
http://adsabs.harvard.edu/abs/2017MNRAS.471.1671D