For many years, we have known that the amount of dark matter at the centres of nearby dwarf galaxies appears to be less than models predicted. This has generated significant interest because it might point to dark matter being more complex than the “cold” thermal relic particle assumed so far.
Recent observations appear to point to a particular solution to the above puzzle: “dark matter heating”. Repeated inflow and outflow of gas during star formation causes the inner gravitational potential to fluctuate with time, causing the dark matter particle orbits to migrate outwards. In this model, we expect an anticorrelation between the inner dark matter density in dwarf galaxies and the amount of star formation they have experienced. This indeed appears to be the case.
However, a puzzle remains. There is mounting evidence for a low dark matter density also at the centres of at least some of the very smallest dwarf galaxies in the Universe: so-called “ultra-faint” dwarfs. These have formed so few stars that “dark matter heating” models do not predict any appreciable lowering of their inner density. In our EDGE project, we set out to run some of the highest resolution simulations of ultra-faints dwarf galaxies to date to address this problem (see Figure). Run on DiRAC, these simulations reach a mass resolution of ~100 solar masses per simulation particle, and a spatial resolution of ~10 light years. A key novelty is the use of “genetically engineered” initial conditions. These allow us to forensically change and explore the impact of late/early assembly on these dwarfs in their full cosmological context. Using this, we found a rather surprising result: late forming ultra-faints were continuously lowering their inner dark matter densities even long after star formation ceased. We tracked this down to a whole new mechanism that can drive dark matter heating in dwarfs: minor mergers. Late assembling dwarfs experience many mergers throughout their lifetimes. As these pass close to the centre on plunging orbits, they can also fluctuate the potential and excite dark matter heating. This could, then, explain the puzzlingly low dark matter density at the centres of some ultra-faint dwarfs.
Project P.I. : Prof. Justin I. Read, University of Surrey
Authors (lead author in bold): Orkney, Matthew D. A. (Surrey); Read, Justin I. (Surrey); Rey, Martin P. (Lund); Nasim, Imran (Surrey); Pontzen, Andrew (UCL); Agertz, Oscar (Lund); Kim, Stacy Y. (Surrey); Delorme, Maxime (CEA, France); Dehnen, Walter (ZAH, Heidelberg)