Galactic angular momentum

Galactic angular momentum flows in EAGLE

We studied the angular momentum of galaxies using the output of the EAGLE simulation (Schaye et al. 2015). Galaxy formation theories suggest that the angular momentum of gas, and by extension stars, is set by interaction with the Dark Matter (DM) halo during formation. The DM halo's angular momentum is explained by tidal torque theory suggesting a powerlaw relation between the specific angular momentum and the mass (j-M) with a power of 2/3. This relation also arises in the EAGLE simulation. Interestingly, observations and simulations have suggested that the stellar angular momentum (j*-M*) has a similar relation to the stellar mass. This is surprising since the stellar and DM halo masses do not have a one-to-one relation. Therefore, the angular momentum of stars is not purely generated through interaction with the DM halo but an additional process must play a role.

Galactic mass evolution

The hypothesis is that the extra process influencing the angular momentum of stars is the ejection and reprocessing of gas, which then forms stars. The figure on the left gives an indication of where the mass involved in the formation of a galaxy is at any given time. A large fraction of the gas will be ejected at some point.

The effect of ejection on angular momentum

We found that gas that is ejected gains angular momentum relative to the gas that remains. This is the case for both ISM and CGM ejection. During ejection the gas interacts with gas at higher radii increasing its angular momentum. The mass dependence of this angular momentum ejection is promising for explaining the j*-M* relation. Further research requires a higher time resolution to further dissect the ejection process. We did not find a clear difference in specific angular momentum of material that is first accreted on to a galaxy versus material that has been processed by another galaxy pre-accretion.