EPSAM

Stellar Dynamics

Lead Supervisor:  Dr Dean McLaughlin

Observations over the past decade with the Hubble Space Telescope have revealed the existence of distinct, compact stellar components at the centres of a large majority of galaxies with masses less than about 10 billion suns. This applies to all types of galaxies, ranging from bulgeless disks through to lenticulars and ellipticals. With the high resolution of space-based observatories, these "stellar nuclei" are resolved, in galaxies up to tens of Mpc distant, into dense clusters of apparently normal stars. We have only begun to understand the precise formation histories of these nuclear star clusters, their relationships to the global stellar populations in their host galaxies, their dynamical states, and the possible connection with the supermassive black holes that occupy the centres of more massive galaxy bulges. One completely unexpected but highly significant observational result is the discovery that the stellar nuclei in early-type galaxies specifically follow an "M-sigma" scaling relation between the mass (M) of the nuclear cluster and the velocity dispersion (sigma) of the parent galaxy. This is parallel to but offset from the well-known "M-sigma" relation that connects the masses of central supermassive black holes to the global velocity dispersions of galaxies. It is generally agreed that the M-sigma relation observed for black holes is the result of radiation-driven feedback that limits gas accretion rates and ultimately stops the growth of black holes and simultaneously chokes off star formation in the galaxies around the black holes. Similarly, a simple model has recently been developed to explain the M-sigma relation for nuclear star clusters in terms of feedback from massive-star winds and supernova explosions in clusters forming at the centres of galaxies. This project is to develop further the feedback explanation for the M-sigma relation (for stellar nuclei specifically, but connecting explicitly to the black-hole relation), and to address some of the more general unanswered questions about nuclear star clusters. With respect to feedback and M-sigma, theoretical models will be constructed that describe the feedback process in more realistic galaxy potentials than have so far been considered (with non-isothermal structures, realistic dark-matter distributions and spatially varying baryon fractions). This will also result in new predictions for more detailed relationships between "central massive objects" (star clusters or black holes) and global galaxy properties. The project will also build self-consistent, equilibrium dynamical models for the stellar nuclei in galaxies. The particularly important question, of why there are no stellar nuclei in galaxies MORE massive than about 10 billion solar masses, will be tackled directly. This work, and comparisons with existing and forthcoming HST observations of nuclear ages, metallicities, sizes and correlations with galaxy properties, will shed new light on the formation histories of the clusters and the nature of their relationship to central supermassive black holes.