PI: Prof. Justin I. Read
In the EDGE project – Engineering Dwarfs at Galaxy Formation’s EDGE – we simulate the smallest stellar systems at an unprecedented spatial resolution of just ~10 light years. At this resolution, we resolve the formation of realistic, dense, star clusters through to the present day, in their full cosmological context, for the first time.
Our latest simulations push to higher mass dwarfs than we have modelled before, reaching a mass where galaxy formation can survive “reionisation” – ionising photons from the first galaxies and quasars – and form stars continuously through to the present day. Raising the mass, we encountered an unexpected surprise: the emergence of dense Nuclear Star Clusters (NSCs). These form through a rather novel mechanism (see Figure 1). When galaxies have their star formation shut down by reionisation, they still retain a significant reservoir of gas that is too hot to form stars. If two of these galaxies undergo a major merger sometime after reionisation, this gas can rapidly cool leading to a massive starburst. In this starburst, many dense star clusters form, spiralling to the galactic centre to form an NSC.
Our new NSC formation mechanism is interesting because it naturally predicts that NSCs, formed in this way, should have two stellar populations separated in age by about a billion years (the time between reionisation and the major merger). This leads to a very distinctive signature in the “colour-magnitude” diagram of the NSC stars which has been seen in about 11 dense star clusters orbiting the Milky Way, but which until now has remained unexplained. This suggests that these star clusters are actually NSCs accreted along with their host dwarf galaxies. If correct, each one should contain dark matter – a remnant of the dwarf galaxy in which they were born. Since most are extremely close to us in astronomical terms – some are just ~15,000 lightyears away – they could be the very best places in the Universe to study the nature of dark matter. Being so close, if dark matter particles annihilate with one another, or decay to produce photons, we would see their radiation much more clearly from these extremely close-by objects than from anywhere else. We are currently digging into the data to see if any hint of dark matter can be found in these NSC-candidate objects.
Read the paper here: https://arxiv.org/pdf/2405.19286