Extragalactic globular cluster systems as dynamical probes of galaxy formation and interaction

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Much of our current understanding of the nature and origin of galaxies depends on our careful analyses of the rich, supremely beautiful globular star clusters. The globular clusters contain the oldest known stars, and many of them date from the time of the formation of the galaxies themselves.  They are thus ideal 'fossil tracers' of a galaxy's  history.  A typical cluster contains hundreds of thousands of sun-like stars held in a mutual gravitational embrace, like a swarm of luminous bees.  Individual globular clusters are consequently ultra-luminous, and can be studied in remote galaxies.  (Indeed, many of the target galaxies are at distances of tens of millions of light years.)  Globular clusters orbit their parent galaxies on timescales of tens or hundreds of millions of years, and their orbital properties and chemical compositions can be studied spectroscopically, using the largest reflecting telescopes on the ground or in Earth orbit. In the early universe, the primordial gas out of which the stars would later form consisted almost entirely of the lightest elements: hydrogen and helium. We investigate the spectroscopic evidence for how much the stars within the globular clusters have changed in composition since the early stages of the universe, and seek to understand how that is related to their dynamical states of motions around the parent galaxies.    In recent years, discoveries of relatively young globular clusters have led to a fresh understanding of how interactions and mergers between entire galaxies can play an important role in galaxy birth and evolution.  The fate of our own Milky Way galaxy, for example, seems inevitably to include a merger with the nearby Andromeda spiral galaxy -- an event a billion years in the future which may be heralded by the spawning of large numbers of rich star clusters.