The Archaeology of Galaxies/Compact Stellar Systems/Anomalous Stellar Populations in GCs

Supervisor: Dr. Mark Norris

The Archaeology of Galaxies

Understanding galaxy formation remains a key goal of astrophysics. Unfortunately, achieving this is hindered by our inability to resolve individual stars in all but the nearest galaxies. This has prevented us from reconstructing the star formation and chemical enrichment histories of external galaxies with detail remotely comparable to what we can achieve for our own Milky Way. To date the best that we can do is to estimate average, light-weighted, ages and metallicities of the stellar populations of external galaxies from their integrated light. This has made it very difficult to understand how the diverse array of galaxy types we see came to be.

This PhD will involve being part of a team to develop and apply a newly developed approach (see that will for the first time allow us to determine the star formation, chemical enrichment, and accretion histories of external galaxies. The derived measurements will then be compared to predictions of a series of state-of-the-art simulations of galaxy formation to allow us to better understand the galaxy formation process. 


Compact Stellar Systems

Massive, extremely dense Ultracompact Dwarfs (UCDs) and Globular Clusters (GCs) are amongst the oldest surviving gravitationally bound stellar systems. As such they provide powerful insights into the earliest periods of the formation and evolution of galaxies. Our work has demonstrated that UCDs are a composite population comprised of the most massive star clusters formed during major galaxy mergers as well as a population of former galaxy nuclei which were released when their original galaxies were destroyed during minor galaxy mergers. Hence these objects can provide insights into assembly history of galaxies.

Several potential projects related to the study of these interesting objects include:

  1. Searches for supermassive black holes in the centres of those UCDs thought to be stripped galaxy nuclei (see e.g.

  2. Examination of the stellar populations of compact stellar systems to see if they share unusual chemical abundance variations only seen in Milky Way globular clusters. This provides a probe of the formation and evolution of stars in the most extreme environments.

  3. The combination of dynamical and stellar population information to use UCDs as definitive probes of the existence of a variation in the Initial Mass Function of stars from galaxy to galaxy.

Anomalous Stellar Populations in GCs

Globular clusters are generally assumed to be the simplest stellar populations, with all stars formed at the same time from the same gas cloud and therefore having the same chemical composition. Because of this they have been used as laboratories to test our theories of stellar evolution, and in particular our models of how the light output of stellar populations change over time. However it has become clear that GCs are not as simple as first thought, and the integrated light of GCs will be affected by unusual stellar populations such as blue straggler stars (stars that appear too young to have formed in the cluster – likely as the result of lower mass stars merging or exchanging mass) and stars with anomalous chemical abundances. This project will use observational and theoretical data to attempt to build models that better capture the integrated light of globular clusters, and which could then be used to more accurately determine the properties of GCs located in other distant galaxies, where individual stars cannot be measured.

These (and other) projects will use the worlds largest optical/IR telescopes (e.g. SALT/LBT/Keck/VLT/Gemini) to provide an unprecedented view of these extreme objects.

If interested in this or other projects related to the study of the formation and evolution of galaxies please contact Dr. Mark Norris directly ( for more information.