A stream come true: Connecting tidal tails, shells, streams, and planes with galaxy kinematics and formation history

Abstract

Context. The rapidly improving quality and resolution of both low surface brightness observations and cosmological simulations of galaxies enable us to address the important question of how the formation history is imprinted in the outer unrelaxed regions of galaxies, and to inspect the correlations of these imprints with another tracer of galaxy formation, the internal kinematics. Aims. Using the hydrodynamical cosmological simulation called Magneticum Pathfinder, we identified tidal tails, shells, streams, and satellite planes, and connected them to the amount of rotational support and the formation histories of the host galaxies. This presents the first combined statistical census considering all these four types of features in hydrodynamical cosmological simulations. Methods. Tidal features were visually classified from a three-dimensional rendering of the simulated galaxies by several scientists independently. Only features that were identified by at least half of the participating individuals were considered to be existing features. The data on satellite planes and kinematic properties of the simulated galaxies were taken from previous work. The results were compared to observations, especially from the MATLAS survey. Results. Generally, prominent features are much more common around elliptical than around disk galaxies. Shells are preferentially found around kinematically slowly rotating galaxies in both simulations and observations, while streams can be found around all types of galaxies, with a slightly higher probability to be present around less rotationally supported galaxies. Tails and satellite planes, however, appear independently of the internal kinematics of the central galaxy, indicating that they are formed through processes that have not (yet) affected the internal kinematics. Prolate rotators have the overall highest probability to exhibit tidal features, but the highest likelihood for a specific type of feature is found for galaxies with kinematically distinct cores (KDCs), nearly 20% of which exhibit streams. Conclusions. As shells are formed through radial merger events while streams are remnants of circular merger infall, this suggests that the orbital angular momentum of the merger event plays a more crucial role in transforming the host galaxy than previously anticipated. The existence of a shell around a given slow rotator furthermore is a sign of a radial merger formation for this particular slow rotator because one-third of the galaxies with a shell were transformed into slow rotators by the merger event that also caused the shells. The appearance of a stream around a KDC is a direct indicator for the multiple merger formation pathway of that KDC as opposed to the major merger pathway.