Angular Momentum Acquisition in Galaxy Halos

TL;DR

This study uses high-resolution simulations to show that cold gas accreted by galaxy halos has significantly higher angular momentum than dark matter, mainly due to filamentary accretion, explaining the formation of cold flow disks.

Contribution

It reveals that cold mode accreted gas enters halos with higher specific angular momentum than dark matter, driven by filamentary accretion, and explains the formation of cold flow disks.

Findings

Cold accreted gas has ~70% higher specific angular momentum than dark matter.

Accreted matter has a higher spin parameter at accretion (~0.1).

Cold flow gas forms disks due to high angular momentum and short residence time.

Abstract

We use high-resolution cosmological hydrodynamic simulations to study the angular momentum acquisition of gaseous halos around Milky Way sized galaxies. We find that cold mode accreted gas enters a galaxy halo with ~70% more specific angular momentum than dark matter averaged over cosmic time (though with a very large dispersion). In fact, we find that all matter has a higher spin parameter when measured at accretion than when averaged over the entire halo lifetime, and is well characterized by \lambda~0.1, at accretion. Combined with the fact that cold flow gas spends a relatively short time (1-2 dynamical times) in the halo before sinking to the center, this naturally explains why cold flow halo gas has a specific angular momentum much higher than that of the halo and often forms "cold flow disks". We demonstrate that the higher angular momentum of cold flow gas is related to the fact that it tends to be accreted along filaments.