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

This is a pre-copyedited, author-produced PDF of an article accepted for publication in Monthly Notices of the Royal Astronomical Society following peer review. The version of record [Philip Taylor, Christoph Federrath, and Chiaki Kobayashi, ‘The origin of kinematically distinct cores and misaligned gas discs in galaxies from cosmological simulations’, Monthly Notices of the Royal Astronomical Society, Vol. 479(1): 141-152, Sept 2018] is available online at: https://doi.org/10.1093/mnras/sty1439. © 2018 The Author(s). Published by Oxford University Press on behalf of the Royal Astronomical Society.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 active galactic nucleus (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 (≈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 ~1/10) 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 underappreciated in the past, and large, high-resolution cosmological simulations will be necessary to gain a better understanding in this area.Peer reviewe