EAGLE simulation shows gain & loss of galaxies’ angular momentum
29 September 2016
Angular momentum is a fundamental property of galaxies, together with mass and energy. It is crucial to many scaling relations, for example the relation between a galaxy’s luminosity and its rotational velocity and size. Galaxy formation theory postulates that the amount of angular momentum in spiral galaxies can be obtained by assuming that they formed in dark matter halos through conservation of angular momentum. Elliptical galaxies though, which have much lower spins, need to lose more than 90% of the angular momentum they were formed with. Galaxy mergers are the main scenario invoked to explain such a major loss.
In a new publication, CAASTRO member Dr Claudia Lagos (ICRAR-UWA) and colleagues analysed the evolution of the angular momentum of galaxies in the EAGLE hydrodynamical simulations. EAGLE is a state-of-the-art simulation that has a unique compromise between the resolution required to study the structural properties of galaxies (spatial resolution of 700 pc) and the simulated cosmological volume (100 Mpc box side length). This allows for the study of about 13,000 galaxies in the simulation-equivalent of the local Universe. EAGLE is unique in its accurate reconstruction of galaxy properties across multiple research studies, predicting galaxies of roughly the right sizes, morphologies, colours, gas contents and star formation throughout cosmic time.
This new study has found a correlation between the galaxies’ specific angular momentum (i.e. angular momentum as function of mass) and their stellar mass – in excellent agreement with observations and with the positions of galaxies as they correlate with gas content.
Analysing galaxy evolution in EAGLE paints a picture that is more complex than what theory predicted: galaxies that have high specific angular momentum now formed most of their stars during the second half of the age of the Universe, from gas that was falling into their halos with high specific angular momentum. In contrast, galaxies that have low specific angular momentum now formed most of their stars during the first half of the age of the Universe, from material that had much lower specific angular momentum compared to the infalling gas later. The researchers conclude that the simple picture of two alternative scenarios – conservation of specific angular momentum or mergers that spin-down galaxies – does not capture what EAGLE has revealed to happen. How quickly a galaxy spins appears to depend on the individual star formation history with a contribution from the merger history.