Action-based Dynamical Modeling of Dispersion-dominated Systems
Creators: name: Gherghinescu, Paula Andreea
Publisher: name: University of Surrey
Year: 2025
Subjects: galactic dynamics, stellar halos, actions, dark matter

description: Dispersion-dominated systems are ubiquitous in the Universe. From large ellipitcal galaxies, to stellar halos and globular clusters, they help us understand various astrophysical phenomena where the behavior of particles or materials is influenced by random motion rather than ordered flow. In this thesis, we focus on two case studies of such systems:
description: stellar haloes of M31-like galaxies and Omega Centauri-like globular clusters.
description: In this thesis, we develop and test action-based dynamical models for dispersion dominates systems, and apply them to two case studies: stellar halos of M31–like galaxies and Omega Centauri-like globular clusters. The dynamical models assume a total axisymmetric potential and a tracer population embedded in this potential and described by an action-based distribution function (DF). The model’s parameters are fitted for in a fully Bayesian framework.
description: In the first part of the work, published in Gherghinescu et al. (2023), we develop and test an action-based dynamical model designed for M31-like galaxies in order to constrain the total galactic potential and the phase space distribution of the stellar halo tracers. The stellar halo component of a galaxy plays a crucial role in understanding the formation and evolution of galaxies because they provide a ”fossil record” of a galaxy’s formation and evolutionary history. We apply the model both mock galaxies and cosmological simulations
description: assuming full 6D phase space knowledge of the data. The results show that the model is robust and recovers the galactic potential well. The total enclosed mass is recovered with a fractional error of around 10% or less out to 100 kpc. The phase space DF of the stellar tracer population is also well recovered except for the highly non-phase mixed and non-equilibrium substructures.
description: In the second part of the thesis, we extend the previous chapter’s work and adapt it to degraded data. The information content and biases incurred due to reduced phase space information availability of the stellar tracers and inclination effects are investigated. We conclude that to probe the total galactic potential of external galaxies it is preferred to use stellar tracers which are part of dispersion-dominated and more spherical structures as opposed to disk stars unless the disk has an edge-on orientation.
description: Finally, in the last chapter, we adapt the dynamical models to Omega Centauri globular cluster-like structures to study the possibility of it hosting a central intermediate-mass black hole (IMBH). We also investigate the data quality necessary to distinguish its dynamical signatures from that of a cluster of stellar remnants. For the mock clusters investigated we conclude that a number of stars of the order of N∗ ≥ 105, with accurate proper motions and line-of-sight velocities, is needed. For fewer stars, especially near the region of influence of the IMBH, no signature is detected which would lead to the false conclusion that no dark central component is present.