Galactic disks are observed to have specific angular momentum contents similar to expectations for typical dark matter halos in $\Lambda$CDM. Cosmological hydrodynamical simulations have only recently reproduced this similarity in large galaxy samples thanks to the inclusion of strong galactic winds, but the exact mechanism by which this is achieved is yet to be clarified. Here we present an analysis of key aspects contributing to this relation: angular momentum selection and evolution of Lagrangian mass elements as they accrete onto dark matter halos, condense into Milky Way-scale galaxies, and become part of the $z=0$ stellar phase. We contrast this evolution in the Illustris simulation with that in a simulation without galactic winds, where the final $z=0$ angular momentum is $\approx0.6$ dex lower. We find that galactic winds give rise to differences between these two simulations in several distinct ways: angular momentum gain, prevention of angular momentum loss, and $z=0$ stars sampling the accretion-time angular momentum distribution of all baryons in a biased way. In both simulations, gas loses on average $\approx0.4$ dex between accreting onto halos and first accreting onto central galaxies. In Illustris, this is followed by $\approx0.2$ dex gains in the `galactic wind fountain’ and no further net evolution past the final accretion onto the galaxy. Without feedback, further losses of $\approx0.2$ dex occur in the gas phase inside the galaxies themselves. An additional $\approx0.15$ dex difference arises due to feedback preferentially selecting gas with higher angular momentum at accretion by expelling gas that is poorly aligned. These and additional effects of similar magnitude are discussed, suggesting a complex origin to the close similarity between the specific angular momenta of galactic disks and of typical halos.
D. DeFelippis, S. Genel, G. Bryan, et. al.
Tue, 14 Mar 17
Comments: 13 pages, 10 figures. Key figures are 1, 2, and 3. Submitted to ApJ