Gravitational torques dominate the dynamics of accreted gas at z > 2

Galaxies form from the accretion of cosmological infall of gas. In the high redshift Universe, most of this gas infall is expected to be dominated by cold filamentary flows which connect deep down inside haloes, and, hence, to the vicinity of galaxies. Such cold flows are important since they dominate the mass and angular momentum acquisition that can make up rotationally-supported discs at high-redshifts. We study the angular momentum acquisition of gas into galaxies, and in particular, the torques acting on the accretion flows, using hydrodynamical cosmological simulations of high-resolution zoomed-in haloes of a few $10^{11}\, \rm M_\odot$ at z = 2. Torques can be separated into those of gravitational origin, and hydrodynamical ones driven by pressure gradients. We find that coherent gravitational torques dominate over pressure torques in the cold phase, and are hence responsible for the spin-down and realignment of this gas. Pressure torques display small-scale fluctuations of significant amplitude, but with very little coherence on the relevant galaxy or halo-scale that would otherwise allow them to effectively re-orientate the gas flows. Dark matter torques dominate gravitational torques outside the galaxy, while within the galaxy, the baryonic component dominates. The circum-galactic medium emerges as the transition region for angular momentum re-orientation of the cold component towards the central galaxy’s mid-plane.