The Star-Formation History of Galaxies Across a Hubble Time

The majority (if not all) of stars are formed in star clusters, yet only a very small fraction of stars today are seen in clusters. What happened? The answer lies in the fact that star clusters do not live forever but instead are slowly disrupted during their lives. Nevertheless, recent advances in our understanding of cluster dissolution have allowed us to 'role back the clock' on whole cluster populations and determine when and how vehemently a galaxy has formed its star clusters (and hence its stars). We have carried out such studies on a handful of nearby galaxies and have accurately traced each galaxies star-formation history for the last billion years. However, a billion years is a mere fraction of a galaxy's life. This limit was an imposed technical limitation, caused by the necessity in this work to have observations in very blue (ultra-violet) light. This limit is about to vanish, due to the next generation of cameras onboard the Hubble Space Telescope. The main new camera, the Wide Field Camera 3 (WFC3) is approximately ten times more efficient in the UV than its predecessor, the Wide Field Planetary Camera 2 (WFPC2). With this new camera we will be able to probe galaxies ten times deeper than previously possible, allowing us an unprecedented view of a cluster population within a galaxy. More importantly however, it will allow us to probe the star-formation history well past the one billion year boundary. Thus, for the first time we will be able to probe the (almost) full history of star-formation within galaxies, from their births to the present epoch. I have been involved in a number of projects in recent years which have sought to use star clusters to derive the SFH of galaxies. While straightforward in principle, a number of pitfalls plague such work, namely observational biases (i.e. detection limits), cluster disruption, and artifacts in the cluster age-finding algorithms. After extensive testing and modeling the robust tools needed are now in hand. Thus, we are in an ideal position to take advantage of the new advances in technology, namely the WFC3. Additionally, deep ground based UV images combined with archival spaced based imaging could be an additional way around the existing technical limits that has not yet been fully explored. In addition to the primary science goal a number of other important questions can be addressed with the current research plan. 1) Determine the size-distribution of star clusters and test for environmental dependence or universality, a key question remaining in star cluster formation theory. 2) Study the clustering properties of different stellar populations (i.e. young and old stars), a continuation of the work that I am presently involved in. 3) Test stellar evolutionary models by comparing the properties (i.e. colour-magnitude diagrams) of the observed stars with models. 4) Quantify the rate, along with any mass or environmental dependence, of 'infant mortality' (the phenomenon where almost 90 percent of clusters are destroyed within the first 20 Myr of their lives. 5) Provide the most stringent tests to date of cluster destruction models.