The origin of kinematically distinct cores and misaligned gas discs in galaxies from cosmological simulations

TL;DR

This study uses cosmological simulations to explore the origins of kinematically distinct cores and misaligned gas discs in galaxies, revealing that gas accretion and minor mergers play key roles in their formation.

Contribution

It demonstrates that atypical galaxy kinematics can result from gas accretion and minor mergers, highlighting mechanisms different from previous models.

Findings

Atypical kinematics are not affected by projection effects.

Counter-rotating gas discs form from accreted gas aligning with cosmological filaments.

Minor mergers can produce kinematically distinct cores (KDCs).

Abstract

Integral field spectroscopy surveys provide spatially resolved gas and stellar kinematics of galaxies. They have unveiled a range of atypical kinematic phenomena, which require detailed modelling to understand. We present results from a cosmological simulation that includes stellar and AGN feedback. We find that the distribution of angles between the gas and stellar angular momenta of galaxies is not affected by projection effects. We examine five galaxies ($\approx 6$ per cent of well resolved galaxies) that display atypical kinematics; two of the galaxies have kinematically distinct cores (KDC), while the other three have counter-rotating gas and stars. All five form the majority of their stars in the field, subsequently falling into cosmological filaments where the relative orientation of the stellar angular momentum and the bulk gas flow leads to the formation of a counter-rotating gas disc. The accreted gas exchanges angular momentum with pre-existing co-rotating gas causing it to fall to the centre of the galaxy. This triggers low-level AGN feedback, which reduces star formation. Later, two of the galaxies experience a minor merger (stellar mass ratio $\sim1/10$) with a galaxy on a retrograde orbit compared to the spin of the stellar component of the primary. This produces the KDCs, and is a different mechanism than suggested by other works. The role of minor mergers in the kinematic evolution of galaxies may have been under-appreciated in the past, and large, high-resolution cosmological simulations will be necessary to gain a better understanding in this area.