Research interests of the group:
Alternative gravity theory and dark matter
Numerous observations from radio to X-ray imply the presence in galaxies, galaxy groups and clusters
of much more dark matter than directly detectable luminous material. What is the nature of this ubiquitous
dark matter? How is it distributed? These are the two theme questions addressed in my research. They are
important for understanding structure formation, which is driven by the common gravitational field of the
dark and luminous matter.
Several lines of attack are used here, combining constraints from galaxy dynamics and gravitational lensing.
First, microlensing of LMC, SMC and Galactic bulge/bar stars can be used to constrain MACHOs in the halo, disk
and bar of the Milky Way. Second the Galaxy's gravitational potential can be mapped out or at least constrained
by studying the motion, orbital decay and tidal distortion of its satellites (e.g., LMC, SMC, Sagittarius and
Ursa Minor dwarfs.) The figure shows a simulation of the encounter between the Sagittarius dwarf and the
Magellanic Clouds.
Gravitational lensing provides a powerful technique for probing the dark matter halo in distant galaxies.
By modelling the images and time delays of lenses (e.g., the quadruple-imaged lens PG1115+080), I hope to
gain greater understanding of the dark matter density, especially the central cusp, in these systems. The
technique will also be able to check whether the lensing potentials are consistent with the popular
Cold Dark Matter cosmology.
Active Galactic Nuclei
The figures are simulations of an Active Galactic Nucleus (AGN) disc with a spiral density wave,
as seen in the light of certain important emission lines. The first frame shows how the disc actually
looks, a view we have never seen because it is too compact - a mere 50 light-days across - to be resolved
even with the largest optical telescopes. Spiral density waves such as these are seen in the accretion
discs of close binary star systems. They are also expected to arise in the accretion disc of an AGN when
its host galaxy merges with a second galaxy possessing a nuclear black hole. Such mergers are thought to
happen repeatedly during the life of a galaxy.
The second frame is a velocity (x-axis) - delay (y-axis) map of the same disc showing the speed and distance
from the ionising source of the gas seen in the Ly alpha 1215 A (red), CIV 1550 A (green) and HeII 1640 A
(blue) lines respectively. The HeII 1640 A emission is coming from material orbiting the black hole at ~
6000 km/sec - a full width of 12,000 km/sec in this image. Such maps can be recovered by the technique of
echo (or reverberation) mapping , which was pioneered at St Andrews and which we continue to develop.
The method works by recording small time-delayed "echoes" in the velocity profiles of photoionised emission
lines. These emission line changes are driven by ionising radiation from the hot inner regions of the accretion
disc. Observational material for echo mapping is currently obtained via major campaigns with HST's ultraviolet
spectrographs in conjunction with various X-ray satellites and ground-based optical telescopes. More information
about echo mapping of AGN discs.
Theoretical and Observational Cosmology
The main research interest is Theoretical and Observational Cosmology, in particular studying Dark Matter
and Dark Energy with Gravitational Lensing, the nature of the Dark Energy, the analysis of Large-Scale
Structure in Galaxy Redshift Surveys, the temperature and polarization of the Cosmic Microwave Background,
and the Early Universe and Cosmological Inflation.