We have been applying our genetic modification technique which involves generating multiple, slightly different sets of early-Universe conditions from which a given galaxy, halo, or void will emerge. As each version of the Universe is evolved in its own computer simulation, the initial differences lead to contrasting evolutions – for instance, the galaxy might be formed earlier or later in the Universe's history, or undergo a different number of mergers with other galaxies. All this takes place in a fully cosmological setting, replicating the accretion of gas and dark matter along filaments.
As such, we provide a highly complementary account of galaxy formation, making clear what the causal relationships are between the history of particular galaxies and their observable properties. Our main results so far systematically probe how galaxy mergers can lead to ‘quenching’, i.e. galaxies that experience a sufficiently major merger cease forming stars and subsequently change their appearance over time. We have connected these mechanisms to the way in which gas gathers around galaxies in the so-called ‘circumgalactic medium’, and proposed novel observational tests which will shed further light on how galaxies of different masses interact with these surroundings.
We have also been developing the genetic modification technique to enable the construction of further tests. We have shown that we can alter not just the mass ratio of mergers, but also the angular momentum of material that falls into our galaxies. At the end of this reporting period, we obtained results showing how angular momentum causally determines the extent of the disk of galaxies, results which will be published shortly and which we will continue to develop. We have created a proof in principle to show that we can re-simulate single galaxies placed into different cosmological environments to see how environments affect galactic evolution.
Finally, we have developed new interpretable machine learning techniques to study the connection between dark matter halos around galaxies, their environment, and the early universe initial conditions.
