Research

My present research interests fall mainly into the category of "near field cosmology". I aim to understand the variety of processes that have driven galaxy formation and mass assembly across cosmological history via the resolved (star-by-star) study of nearby systems - in particular the Milky Way and Andromeda (especially their stellar halos and globular cluster populations), the Magellanic Clouds, and the myriad of smaller dwarfs that inhabit the "Local Group".á 

This forensic dissection of galaxies is sometimes called "galaxy archaeology" because we use the clues encoded in the observable properties (locations, dynamics, ages, elemental abundances) of different stellar populations to look back in time and infer sequences of events, ambient conditions, and the relative importance of various physical processes involved in producing the systems seen today. This information can then be compared with the predictions from cosmological models of galaxy formation.

I am a member of the Pan-Andromeda Archaeological Survey (PAndAS) collaboration. We have used deep wide-field imaging to map the remote outskirts of our nearest large neighbour, the Andromeda galaxy (M31). This represents the most complete view ever assembled of the stellar halo around a large galaxy, including the Milky Way. The halo of Andromeda is littered with stellar streams and over-densities, indicating that the accretion and disruption of smaller (dwarf) galaxies has been important in building up the larger system. We are now using the stellar populations in these streams to learn about the properties of the progenitor systems, and the stream kinematics to infer where these small galaxies came from, and how long ago, as well as the nature of the underlying gravitational potential of the Andromeda halo (i.e., how much unseen mass is there and how it is distributed).

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We have used the PAndAS data to uncover nearly one hundred previously unknown globular clusters in the outer halo of Andromeda (pink dots above). This is many more than are seen in the outskirts of the Milky Way. The positions of these new globular clusters show a clear correlation with stellar streams in the halo, indicating that they too were most likely accreted from now-defunct dwarfs. This in turn suggests that remote globular clusters might be good signposts for stellar streams in the outer Milky Way, a possibility that we are exploring with the new generation of super-wide-field imagers (e.g., SkyMapper, DECam).

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Globular clusters are fascinating objects in their own right. Most are seen to be very nearly as old as the Universe, and thus offer a rare window into the conditions of star formation at very early times. An enduring mystery is the observation that stars in globular clusters tend to show distinct patterns in the abundances of certain elements - most commonly oxygen, sodium, carbon, nitrogen, magnesium and aluminium. These patterns suggest that globular cluster formation involved a moderately complex sequence of events, although as yet there are no models that can explain these in a physically self-consistent way. One possible avenue for progress is to look at young massive star clusters (YMCs) - objects which might, given time, evolve into systems similar to the ancient globular clusters. The Large and Small Magellanic Clouds host YMCs with a variety of properties, including a subset that appear to exhibit properties suggesting that the processes involved in their formation may have been similar to those inferred for bona fide globular clusters. I am conducting detailed observations to test whether this is a tenable hypothesis, and to explore the potential implications for our understanding of star formation conditions at very early times.

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