The dominant component of matter in the Universe is believed to be dark matter, an unknown subatomic particle that only interacts with ordinary matter via gravity. Through gravitational growth, dark matter forms into so-called halos, within these halos that the visible galaxies we see in the Universe form and grow.
How galaxies form is a complex process, and recent simulations such as SIMBA (run on DiRAC’s cosma-7 in 2019) have demonstrated that widespread ejection of ordinary matter out of halos is a requirement in order to produce simulated galaxies that look like real ones. Even though ordinary matter is only one-sixth of the total mass in the cosmos, large-scale redistribution of ordinary matter can noticeably impact dark matter halos. But when, where, and how much does this happen, in both the ordinary matter and the dark matter?
Using SIMBA, Sorini, Davé et al. (2022, MNRAS, 516, 882) presented the most comprehensive analysis to date of this question within cosmological simulations. The answer depends on both the cosmic epoch and the mass of the halo. At low masses, the process of galactic winds driven by powerful supernova explosions ejects ordinary matter out of low-mass (i.e. smaller than Milky Way-sized) halos, while leaving high-mass halos relatively unaffected. This happens very early on in cosmic time, resulting in low-mass halos have suppressed matter contents compared to their higher-mass counterparts. However, as the Universe progresses, large supermassive black holes grow within massive galaxies that are able to eject material despite the large halo masses. As a result, over the latter half of the Universe, the high-mass halos likewise become matter-deficient.
The dark matter responds by being dynamically dragged by the ejected ordinary matter, which can impact the dark matter profile. By comparing to a SIMBA-like simulation that includes only dark matter, this is seen to have a particularly large impact on the matter power spectrum at intermediate scales. If SIMBA’s results are true, this would imply significant corrections need to be applied to the matter power spectrum from ESA’s soon-to-be-launched Euclid mission to be able to use Euclid’s weak lensing measurements in order to infer cosmological parameters such as the amount of cosmic dark matter and dark energy.