I am a Postdoctoral Research Associate at the Institute of Astronomy at the University of Cambridge and a Junior Research Fellow at Christ's College, Cambridge. Prior to this I read for a DPhil in Theoretical Astrophysics in the Rudolf Peierls Centre for Theoretical Physics at the University of Oxford under the supervision of Prof. James Binney.
I am interested in the dynamics of the Milky Way and the development of techniques to help us understand the structure and formation of the Galaxy. There is a brief description of what I do below. For other information, check out my cv.
milky way dynamical modelling
The motions of the stars in our Galaxy today tell us something about its dynamical state. However, the history of our Galaxy is tied up in the compositions of the stars. These internal properties do not change throughout the life of the stars, whereas a star can move around in the Galaxy considerably during its lifetime.
By extending a fully dynamical distribution function we can include information on the chemical properties of the stars. These models may then be fitted to data, and used to understand the complex mechanisms which have caused the Galaxy to appear the way it does today.
The halo of the Galaxy is rich in substructure. This substructure is the result of The Milky Way accreting material from smaller satellite galaxies. Debris from the satellites is tidally stripped by the Milky Way, forming long tidal streams of stars. These structures are of interest as they reveal something about the gravitational potential of the Galaxy, and hence the underlying dark matter distribution.
I have done some work investigating the best way to find the potential using tidal stream data. The structure of a tidal stream is particularly simple when viewed in angle-action coordinates (see below). These special coordinates are linked to the shape of the gravitational potential of the Galaxy, so the stream will look correct in these coordinates only when we pick the correct gravitational potential.
As streams are so narrow, they provide sensitive probes of the presence of substructure in the Galactic halo. When a dark subhalo passes near a stream, it punches a hole through it producing a visible gap. Our models can incorporate these effects and indeed it appears that the Palomar 5 stream has a level of substructure consistent with that predicted by simulations.
Sanders, Bovy & Erkal, 2016, MNRAS • Sanders, 2014, MNRAS • Sanders & Binney, 2013b, MNRAS • Sanders & Binney, 2013a, MNRAS • Bovy, Erkal & Sanders, 2016, MNRAS • Erkal, Belokurov, Bovy & Sanders, 2016, MNRAS • Erkal, Sanders & Belokurov, 2016, MNRAS
The dwarf spheroidal satellite galaxies of the Milky Way are some of the most dark matter dominated regions of our Galaxy. One natural consequence of some dark matter models is self-annihilation leading to the possibility of observing gamma-ray signals from regions of high dark-matter density. To infer the nature of the dark matter particle from gamma-ray observations, we require knowledge of the quantity and distribution of dark matter in the dwarf spheroidal galaxies.
We have developed methods for robustly inferring the mass of dark matter within flattened galaxies. We have demonstrated how one can simply compute the expected dark-matter annihilation signal and how this can be adjusted to account for the effects of flattening, which gives rise to a small change in the expected hierarchy of annihilation signals from the dwarf galaxies.
To construct models of galaxies and tidal streams, I use dynamical quantities called the action integrals. Each star in the Galaxy feels the gravitational pull of all the other stars, the gas and the dark matter causing it to follow an orbit.
In the movie, there is an example of such an orbit. Although this path looks complicated, it can be simplified by transforming to action coordinates. These are constants of the motion and characterise a torus on which the star moves. The position on the torus is then characterised by the angle coordinates which simply increase linearly in time (depicted in the second movie).
With these quantities we can build up dynamical models. However, although the actions are conceptually simple, in practice they are difficult to calculate. I have developed approximate schemes for their computation and published code that performs these calculations on github.