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Dr Steven Williams

Formerly at Lancaster University

Research overview

I work on observations of high redshift supernovae, particularly spectroscopy of them and/or their host galaxies. The candidates are discovered by a dedicated Hubble Space Telescope survey, which also follows the discovered candidates in order to obtain a light curve. Spectra are then obtained using world-class instruments such as the Very Large Telescope to determine the redshift. The main goal of this project is to increase the constraints on dark energy at high redshifts. I also work on observations of nearby transients, particularly novae occurring in the Local Group of galaxies.

Research Interests

Observations of supernovae in other galaxies began with the supernova of 1885 in the nearby Andromeda Galaxy (M31). It was a further ten years before the next discovery, but as instruments and techniques have improved, more and more have been observed each year (over 3000 candidates were discovered in 2015, with about 25% being confirmed spectroscopically). They are broadly split into types according to the spectra the explosions show. One of these types, Type Ia supernovae, are caused by the explosion of a white dwarf as it reaches a critical mass.

Type Ia supernova are particularly important as they can be used to calculate distances, and because they are so luminous they can be observed out to extreme distances. They were used in the discovery that the expansion of the universe is accelerating (Riess et al. 1998; Perlmutter et al. 1999), work which was awarded the 2011 Nobel Prize in Physics. I am currently working on a project to extend the observations of Type Ia supernovae to even greater distances, focusing on supernovae beyond redshift 1, which are explosions that actually occurred when the universe was less that half of its current age. The work uses the Hubble Space Telescope to discover probable supernovae and we then obtain ground-based spectroscopy using world-class telescopes such as the Very Large Telescope, which can reveal properties of the host galaxies and sometimes even the supernovae themselves. The main aim of this work is to use the supernovae to constrain dark energy at these high redshifts.

I also work on novae, which are explosions occurring on the surface of a white dwarf (rather than the explosion of the white dwarf itself in the case of Type Ia supernovae). The majority of my work is on novae occurring in nearby galaxies such as M31, M33, IC 1613 and M81. This allows us a better view of the statistics of a nova population than observing novae in our own Galaxy does. It has also revealed extreme objects such as M31N 2008-12a, which erupts every year (Darnley et al. 2015; Henze et al. 2015). The main project of my PhD was to search for M31 novae with red giant companion stars, which can be directly imaged with the Hubble Space Telescope. This revealed a surprisingly high fraction of novae appear to harbour red giants and that such novae seem more likely to reside in the M31 disk than the bulge (Williams et al. 2014, 2016).


PhD in Astrophysics at Liverpool John Moores University (2014)

Thesis: Extragalactic Novae and Their Progenitors

Supervisors: Dr Matt Darnley and Prof. Mike Bode

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