The Creation and Persistence of a Misaligned Gas Disc in a Simulated Early-type Galaxy

In a new paper, Freeke van de Voort and collaborators use a simulation of a massive galaxy from the FIRE project to study misaligned gas discs in early type galaxies. The simulation reveals that gas accretion (smooth and via galaxy mergers) can create gas discs that are kinematically misaligned with the stellar disc of the galaxy, and that the misalignement can persist for much longer than predicted by dynamical models of isolated galaxies. The persistence of misaligned gas discs has important implications for connecting observed misaligned disc statistics to the cosmic merger rate.

A movie of the evolution of the gas disc in the simulation can be viewed here.

Abstract: Massive early-type galaxies commonly have gas discs which are kinematically misaligned with the stellar component. These discs feel a torque from the stars, however, and the angular momentum vectors are naively expected to align within a few dynamical times. We present results on the evolution of a misaligned gas disc in a cosmological ‘zoom-in’ simulation of a massive early-type galaxy from the Feedback In Realistic Environments (FIRE) project. This galaxy experiences a merger at z=0.3, which, together with a strong galactic wind, removes most of the gas disc that was in place. The galaxy subsequently reforms a gas disc through accretion of cold gas, but it is initially 120 degrees misaligned with the stellar rotation axis. This misalignment persists for about 2 Gyr before the gas-star misalignment angle drops below 20 degrees. This is about 150 times longer than the dynamical time in the central kpc and varies with galactocentric radius. The time it takes for the gaseous and stellar components to align is much longer than previously thought, because the gas disc is accreting a significant amount of mass for about 1.5 Gyr after the merger, during which the angular momentum change induced by accreted gas dominates over that induced by stellar torques. Once the gas accretion rate has decreased sufficiently, the gas disc decouples from the surrounding halo gas (which remains misaligned) and realigns with the stellar component in about 6 dynamical times, independent of radius. When stellar torques dominate the evolution of the misaligned gas disc, the centre aligns faster than the outskirts, temporarily resulting in a warped disc. We discuss the observational consequences of the long survival of our misaligned gas disc and how our results can be used to calibrate merger rate estimates from observed gas misalignments.