Angular momentum transfer in cosmological simulations of Milky Way-mass discs

ABSTRACT Fuelling star formation in large, discy galaxies requires a continuous supply of gas accreting into star-forming regions. Previously, we characterized this accretion in four Milky Way mass galaxies ($M_{\rm halo}\sim 10^{12}{\rm M}_{\odot }$) in the FIRE-2 cosmological zoom-in simulations. At $z\sim 0$, we found that gas within the inner circumgalactic medium (iCGM) approaches the disc with comparable angular momentum (AM) to the disc edge, joining in the outer half of the gaseous disc. Within the disc, gas moves inwards at velocities of $\sim$1–5 km s$^{-1}$ while fully rotationally supported. In this study, we analyse the torques that drive these flows. In all cases studied, we find that the torques in discs enable gas accreted near the disc edge to transport inwards and fuel star formation in the central few kpc. The primary sources of torque come from gravity, hydrodynamical forces, and the sub-grid $P \mathrm{ d}V$ work done by supernova (SN) remnants interacting with gas on $\lesssim$10 pc scales. These SNe remnant interactions induce negative torques within the inner disc and positive torques in the outer disc. The gas–gas gravitational, hydro, and ‘feedback’ torques transfer AM outwards to where accreting gas joins the disc, playing an important role in driving inflows and regulating disc structure. Gravitational torques from stars and dark matter provide an AM sink within the innermost regions of the disc and iCGM, respectively. Feedback torques are dominant within the disc, while gravitational and hydrodynamical torques have similar significance depending on the system/region. Torques from viscous shearing, magnetic forces, stellar winds, and radiative transfer are less significant.