The emergence of globular clusters, dwarf galaxies & something in-between

Globular star clusters were first discovered by Abraham Ihle in 1665. They are the oldest and densest stellar systems in the Universe, yet we do not know how they formed. To add to the mystery, at the same time as the Universe was busy forming these dense globulars, it also formed the very least dense stellar systems known - dwarf galaxies.

Dwarfs differ from globulars in almost every possible way. At the same brightness, globulars are less than one tenth the size. While dwarfs are almost entirely comprised a mysterious, invisible, substance we call "dark matter", globulars appear to contain none.

And, globular cluster stars show strange correlations between some of their chemical elements, like nitrogen and oxygen, that are not seen in dwarf galaxy stars. None of this is understood. Finally, as new and deeper survey data have uncovered new objects, we are finding lower density globulars and smaller dwarfs that have begun to overlap in their properties, challenging our notions of what even makes the two objects distinct.

The PI leads an international project focussed on simulating the very smallest galaxies in the Universe: EDGE. EDGE has recently passed a key milestone in that it can resolve the impact of individual exploding stars on their surrounding environment. At this new level of realism, realistic dwarf galaxies and globular clusters suddenly emerge from the calculation, for the first time.

In this proposal, we will capitalise on this exciting new theoretical development. We will combine these new results, with detailed follow up calculations and new data obtained by our group to tackle the problem of how globulars form afresh. We are uniquely placed to make progress on this old problem.

Firstly, the EDGE simulations are the first to self-consistently form globulars alongside dwarf galaxies in the Standard Cosmology, opening up the tantalising possibility that we could solve the long-standing problem of how globulars form. Secondly, the simulations make a number of novel predictions that we will test in this proposal to determine if our new model for globular formation is on the right track.

In particular, the simulations predict the existence of a new class of object that look like globulars but contain dark matter, that we call "dark matter clusters". And, they predict that dwarfs and globulars should overlap in size and luminosity, but still be distinguishable from their distinct chemistry. We will confront these predictions with a host of exciting new data for objects that lie in the region where globulars and dwarfs overlap.

In so doing, we will refine our definitions of both "globulars" and "dwarfs", hunt for "dark matter clusters" in the nearby Universe, and make significant progress on the decades-old problem of how globulars form.