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

TL;DR

This study demonstrates that gravitational torques are the primary mechanism influencing the angular momentum and orientation of cold gas inflows in high-redshift galaxies, with implications for galaxy formation and disk development.

Contribution

It reveals that gravitational torques dominate over pressure torques in cold gas accretion at high redshift, highlighting their role in galaxy angular momentum evolution.

Findings

Gravitational torques dominate pressure torques in cold gas flows.

Dark matter torques are significant outside the galaxy, baryonic torques inside.

Circum-galactic medium is key for angular momentum re-orientation.

Abstract

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 halos, 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 disks 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 halos 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.