title: The relationship between the morphology and kinematics of galaxies and its dependence on dark matter halo structure in simulated galaxies
Creators: name: Thob, A
Publisher: name: Liverpool John Moores University
Year: 2019

description: Galaxies are among nature’s most majestic and diverse structures. They can play host to as few as several thousands of stars, or as many as hundreds of billions. They exhibit a broad range of shapes, sizes, colours, and they can inhabit vastly differing cosmic environments. The physics of galaxy formation is highly non-linear and involves a variety of physical mechanisms, precluding the development of entirely analytic descriptions, thus requiring that theoretical ideas concerning the origin of this diversity are tested via the confrontation of numerical models (or “simulations”) with observational measurements. The EAGLE project (which stands for Evolution and Assembly of GaLaxies and their Environments) is a state-of-the-art suite of such cosmological hydrodynamical simulations of the Universe. EAGLE is unique in that the ill-understood efficiencies of feedback mechanisms implemented in the model were calibrated to ensure that the observed stellar masses and sizes of present-day galaxies were reproduced. We investigate the connection between the morphology and internal kinematics of the stellar component of central galaxies with mass M* &gt; 10^9.5 Msun in the EAGLE simulations. We compare several kinematic diagnostics commonly used to describe simulated galaxies, and find good consistency between them. We model the structure of galaxies as ellipsoids and quantify their morphology via the ratios of their principal axes. We show that the differentiation of blue star-forming and red quiescent galaxies using morphological diagnostics can be achieved with similar efficacy to the use of kinematical diagnostics, but only if one is able to measure both the flattening and the triaxiality of the galaxy. Flattened oblate galaxies exhibit greater rotational support than their spheroidal counterparts, but there is significant scatter in the relationship between morphological and kinematical diagnostics, such that kinematically-similar galaxies can exhibit a broad range of morphologies. The scatter in the relationship between the flattening and the ratio of the rotation and dispersion velocities (v/σ) correlates strongly with the anisotropy of the stellar velocity dispersion: at fixed v/σ, flatter galaxies exhibit greater dispersion in the plane defined by the intermediate and major axes than along the minor axis, indicating that the morphology of simulated galaxies is influenced significantly by the structure of their velocity dispersion. The simulations reveal that this anisotropy correlates with the intrinsic morphology of the galaxy’s inner dark matter halo, i.e. the halo’s morphology that emerges in the absence of dissipative baryonic physics. This implies the existence of a causal relationship between the morphologies of galaxies and that of their host dark matter haloes. Using these tools, we also investigate the morphology and kinematics of central galaxies with mass M* &gt; 10^9.5 Msun and their globular cluster (GC) populations in the EAGLE spinoff project E-MOSAICS that incorporates the MOSAICS model of stellar cluster formation and evolution. We find that metal-poor and metal-rich GC populations (split by a threshold at [Fe=H] = -1) exhibit differing characteristic morpho-kinematic properties. The former are significantly more elliptical and dispersion-supported than the latter. In detail, the relations connecting the kinematic properties of the populations with those of their host galaxy exhibit good agreement with available observations, with the velocity dispersions of both metal-poor and metal-rich GC populations correlating positively with that of the host galaxy. The rotational velocity of the metal-rich globular cluster population also correlates positively with that of the host galaxy. The relationship between the morpho-kinematics of the metal-rich globular clusters and the host galaxy’s field stars exhibits a scatter that is sensitive to the relative ages of the two populations: for fixed globular cluster kinematics, the field star population is more disky/rotation-supported if they are younger. We confirm that the connection between the morpho-kinematics of galaxies and their velocity dispersion anisotropy revealed in EAGLE is also exhibit in the higher-resolution E-MOSAICS simulation. However, the same behaviour is not seen in the metal-rich globular clusters, which we speculate is a consequence of them not representing a self-gravitating system.