The creation and persistence of a misaligned gas disc in a simulated early-type galaxy

TL;DR

This study uses cosmological simulations to show that misaligned gas discs in early-type galaxies can persist for over 2 billion years due to ongoing gas accretion, challenging previous assumptions about rapid alignment.

Contribution

It demonstrates that gas accretion can maintain misalignment for extended periods, affecting interpretations of galaxy merger histories and gas dynamics.

Findings

Misaligned gas discs can last over 2 Gyr in early-type galaxies.

Gas accretion dominates angular momentum changes during misalignment.

Warped discs form as the outer regions realign faster than the center.

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 and the angular momentum vectors are expected to align quickly. We present results on the evolution of a misaligned gas disc in a cosmological simulation of a massive early-type galaxy from the Feedback In Realistic Environments project. This galaxy experiences a merger which, together with a strong galactic wind, removes most of the original gas disc. 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. 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 and realigns with the stellar component in about 6 dynamical times. During the late evolution of the misaligned gas disc, the centre aligns faster than the outskirts, 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.