Feeding the Beasts: Shedding light on the formation processes of today's most massive galaxies.

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We have discovered a new - an unexpected - formation mechanism for the most massive galaxies in the universe: Brightest Cluster Galaxies or BCGs.  These systems sit at the centre of rich and massive galaxy clusters, held together by the mutual gravity of their components. Although the standard accepted theory indicates that BCGs should assemble the bulk of their stellar mass through the accretion of smaller gas-poor systems (the so-called hierarchical structure formation scenario) the recent results of my team have shown that substantial in-situ star formation occurs in young BCGs. This is a key result since, as an extreme outcome of evolution, these massive systems are sensitive probes of the models of galaxy formation. It is also surprising as it flies in the face of our understanding of how galaxy clusters accrete galaxies:  during in-fall into the cluster the galaxies should be stripped of their interstellar medium and be gas poor by the time they merge wit the BCG - thus making star formation impossible. Yet the immense amount of star formation in and around BCGs indicate that large amounts of fuel have been deposited into the centres of these galaxy clusters.  Over the next five years, my team will undertake an ambitious observational program to elucidate the nature of the gas deposition and star formation in these systems.  Specifically, our objective is to understand what triggered the intense star formation we have measured, what sustains it and what will eventually quench it.   This will require an approach the spans the electromagnetic spectrum of light as different processes emit light at different wavelengths. We will employ the newly-online and next generation facilities such as the Atacama Large Millimetre Array and the James Webb Space telescope, as well as established workhorses such as the Gemini 8m optical telescopes and the Jansky Very Large (radio) Array.    The program will encompass the three main aspects of galaxy stellar mass assembly:  (i) the current or ongoing star formation; (ii) the existing stars, or the historical star formation and; (iii) the molecular gas fuel, or the future star formation.  This three-pronged approach will clarify for the first time, the formation mechanisms of these massive beasts and provide new constraints for the models and theory which form the basis of our understanding of massive galaxy evolution.