Uncovering and Understanding the Hidden Globular Cluster System of NGC 5128 at Large Radii

Abstract

The Lambda+Cold Dark Matter cosmological model of the universe is successful at replicating the large scale distribution of galaxies, but fails to match observations at smaller, galaxy scales. In order to match theory with observation, the substructure properties of more galaxies with a variety of properties and environments must be measured. Fully understanding the galaxy NGC 5128 will provide a unique counterpoint to galaxies in the Local Group - it is a giant elliptical galaxy in a dense environment with evidence of merger events. In Chapter 2 I describe a new technique that connects data from the Gaia space telescope with ground-based observations to identify globular cluster (GC) candidates associated with NGC 5128 out to R~150 kpc. I present a new catalog of GC candidates, ranked based on the likelihood that they are true GCs and highlighting those candidates that should be the highest priority for spectroscopic follow-up for confirmation. Additionally, I revise the estimate of the total expected GC population size and show that a substantial number of sources previously argued to be low-velocity GCs are instead foreground stars, bringing the mean velocity and velocity dispersion of the GC system more in line with the galaxy as a whole. This work was published in Hughes et al. 2021. In Chapter 3 I verify my GC candidate selection technique by collecting a large sample of spectra using Magellan/M2FS and AAT/AAOmega. I present new radial velocity measurements for 173 previously known and 125 newly confirmed GCs in NGC 5128. I identify GCs with clear kinematic trends that are associated with halo substructure visible in red giant branch star maps of the galaxy. I examine the systemic velocity, the projected rotation amplitude and axis, and the projected velocity dispersion of the GC system as functions of galactocentric radius, metallicity, and substructure association.Using the updated population of GCs as tracer particles, I make new estimates of the total mass of the galaxy at a variety of radii. These findings further indicate that NGC 5128 formed via hierarchical merging over other methods of formation, such as major merging at late times.