Horizon

Initial conditions for galaxy formation simulations are specified within the Λ-CDM scenario by a Gaussian random field. This leads to a troubling lack of control: we often cannot ask precisely what leads to certain outcomes for a galaxy. What is it about a galaxy’s mass, environment or history that gives it its shape or its colour? Imagine we could control the initial conditions of a galaxy in cosmological simulations and tweak them to alter one aspect of its formation while keeping the other aspects the same.

The projected density of dark matter at redshift 3 (top row) and 2.3 (middle row) for three simulations with nearly identical initial conditions, but “genetically modified” to produce slightly different merging histories of the main galaxy in the center of the dark matter images. Bottom row shows the main galaxy in the IVU rest frame colours. The galaxy becomes increasingly quiescent, red and spheroidal as as the merger ratio increases from left to right.

Thanks to the DiRAC facility, our group has run the first generation of high resolution cosmological simulations that alters the “DNA” of a galaxy (Roth et al. 2016, MNRAS, 455, 974; Pontzen et al., arXiv:1608.02507). Roth et al. 2016 sets out the “genetic modification” approach and Pontzen et al. showcases its power (see figure) to understand what quenches star formation in a galaxy. By minimally altering the linear overdensity in the cosmological initial conditions, Pontzen et al changes the merger history of a galaxy while leaving its final halo mass, large scale structure and local environment unchanged. The changing merger history influences the impact of feedback from Active Galactic Nuclei leading to three very different fates for the galaxies: star forming, temporarily quenched, permanently quenched.