title: Using globular clusters to unveil the properties of the light and dark galaxy halo
Creators: name: Hughes, M
Publisher: name: Liverpool John Moores University
Year: 2021

description: Galaxies encode fossil records of their formation histories. While in the Milky Way there is the opportunity to observe individual stars, in other galaxies this is not always possible. Low surface brightness in the outskirts and crowding in the centre makes it challenging to determine the age, chemical composition, kinematics and spatial sub- structure of an external galaxy. Globular clusters however, are collections of stars that are bright and reach far out into a galaxy halo, making them ideal candidates to unveil hidden properties of the galaxy in which they reside, including their formation histories and dark matter distributions. To use GCs as tracers of the formation history of a galaxy, one must first under- stand how these objects form and evolve alongside their host galaxies. The MOd- elling Star cluster population Assembly in Cosmological Simulations within EAGLE (E-MOSAICS) simulations, are a tool to do just that. The simulations are a suite of 25 zoom-in Milky Way-mass galaxies and, more recently, a 34 Mpc volume that follow the co-formation and evolution of galaxies and their GC populations. In this thesis, the simulations are used to address multiple questions of GC and galaxy co-formation. The mass function of globular cluster (GC) populations is a fundamental observable that encodes the physical conditions under which these massive stellar clusters formed and evolved. In this thesis I show that the cluster formation model in E-MOSAICS reproduces the observed shapes of GC mass functions. Furthermore, I examine the origin of the shape and relate it to the mass and the formation history of galaxies, concluding that it is a combination of nature and nurture that sets this shape. Stellar streams are the most direct evidence of accretion onto a galaxy halo. In this thesis I examine the properties of GCs associated with stellar streams. More massive accreted galaxies typically contribute younger and more metal rich GCs. Futhermore, GCs associated with stellar streams have systematically lower [α/Fe] at fixed [Fe/H] than other GCs in a galaxy halo. This lower age results from a more extended cluster formation history in more massive galaxies. In addition, at fixed stellar mass, galaxies that are accreted later host younger clusters, because they can continue to form GCs without being subjected to environmental influences for longer. This explains the large range of ages observed for clusters associated with the Sagittarius dwarf galaxy in the halo of the Milky Way compared to clusters which are thought to have formed in satellites accreted early in the Milky Way’s formation history. The α-element abundances of the globular cluster (GC) and field star populations of galaxies encode information about the formation of each of these components. The [α/Fe]-[Fe/H] distribution of GCs largely follows that of the field stars and can also therefore be used as tracers of the [α/Fe]-[Fe/H] evolution of the galaxy. There is a wide range of shapes for the field star [α/Fe]-[Fe/H] distribution, with a notable subset of galaxies exhibiting bimodal distributions, in which the high [α/Fe] sequence is mostly comprised of stars in the bulge, a high fraction of which are from disrupted GCs, where this fraction correlates with the galaxy’s formation time. Using this result, I suggest that the Milky Way experienced a phase of unusually rapid growth at early times. The E-MOSAICS simulations can also be used to test other models to inform the users about any biases they should be aware of. Dynamical models allow the user to connect the motion of a set of tracers to the underlying gravitational potential, and thus to the total (luminous and dark) matter distribution. Globular clusters (GCs) are an ideal tracer population in dynamical models. This thesis tests how well Jeans-Anisotropic- MGE (JAM) models using GCs (positions and line-of-sight velocities) as tracers can constrain the mass and radial distribution of DM halos. There is a strong correlation between how well we recover the mass and the radial distribution of the DM and the number of GCs in the galaxy: the constraints get exponentially worse with fewer GCs, and at least 150 GCs are needed in order to guarantee that the JAM model will perform well. These results contribute to the growing body of work that recognises the close relation- ship between GC and galaxy formation. This thesis shows clearly that the properties of GCs can be used to investigate the formation history of both the visible and dark components of a galaxy.