Dark matter in Galaxies

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.

I also do modified Gravity theories, which give rise to the effects of Dark Matter without physically introducing them.  They are also motivated by our lack of a fundamental understanding of the Dark Energy phenomena.  Both issues can find answers in theories related to the so-called MOND formulae of galaxies.  I have been expanding my research in this area through many external collaborations in UK, Europe and China.

I have been teaching the following 5 astronomy and physics modules.

Great Ideas ID1004 (for non-astronomy students)

Physical Universe AS1002 (a module for non-astronomy students)

Astronomy and Astrophysics AS2001+AS2101

Observational Cosmology AS4021+AS4010

General Relativity PH5011