The role of angular momentum in the formation of disk galaxies at high redshift

The role of angular momentum in the formation of disk galaxies at high redshift

One of the key factors determining galaxies’ observable properties, such as their morphology and colour, is their angular momentum.

However, the origin of this angular momentum remains uncertain. Observations reveal that the spins of neighbouring galaxies are correlated one with another, a signal known as the intrinsic alignment signal. This suggests that part of this angular momentum should have a common cosmological origin to be elucidated. This effect has so far been studied statistically in large-volume simulations.

Figure 1: Mock images of a simulated galaxy three billion years after Big Bang. We modify how much angular momentum it will accrete, as labelled above. The same galaxy with increased angular momentum shows a more prominent disk structure and a larger spatial extent (as indicated by the white circle). Comparing different realisations of the same galaxy with different formation histories reveals the origin of its angluar momentum.

In addition, angular momentum is correlated with galaxy observables: for example, fast-rotating galaxies are more likely to display spiral structures or host more massive supermassive black holes. As of today, it remains to be confirmed that angular momentum is the underlying driver of these correlations rather than a consequence of another hidden one.

This DiRAC project aims at addressing these questions. To that end, we have performed a suite of cosmological zoom-in simulations using state-of-the-art physical models. We modified the initial conditions of seven galaxies so as to increase or decrease how much angular momentum they will accrete 3 billion years later (for the first three) and 6 billion years later (for the last four). This is achieved by modifying the initial conditions of the simulations (using the genetic modification technique, Roth et al. 2016; and in particular angular momentum modifications, Cadiou et al. 2021). We can thus make sure that there are no extra hidden parameters controlling the angular momentum.

In our first paper (Cadiou et al. 2022), we studied the first three galaxies with modified angular momentum 3 billion years after big bang. We find that this causes a change in the trajectory of their infalling satellites, which subsequently spin up or down the galaxy. We show how the resulting change in stellar angular momentum causes changes to the galaxy’s size and shape, as illustrated in Figure 1.

Future analysis of the four galaxies with modified angular momentum 6 billion years after big bang will allow us reveal whether similar mechanisms govern angular momentum acquisition in the late Universe. We will also concentrate future work on a detailed analysis of the gas trajectory from cosmological scales all the way to the center of the galaxy where stars form.