Dissertation

Constraining Dark Matter with Gravitational Heating and Cooling Processes

Dutta Chowdhury, Dhruba

2022

Overview

About 84% of the total mass budget of the Universe consists of dark matter. While it does not interact electromagnetically and, therefore, cannot be “seen”, one can hope to infer its properties from its gravitational imprints on the structure and dynamics of various astrophysical objects (e.g., star clusters, black holes, galaxies). While cold dark matter, where dark matter is collisionless and most likely consists of weakly interacting massive particles (WIMPs), is the most widely studied dark matter model, WIMPs remain undetected. An intriguing alternative is Fuzzy Dark Matter, where dark matter is made up of ultralight bosons with masses in the range 10-23 eV ≲ mb ≲ 10−19 eV. Unlike in CDM, FDM halos consist of a central core, surrounded by an envelope of order unity density fluctuations. The central core is the ground state of the Schrödinger-Poisson (SP) equation that governs FDM dynamics, and the envelope consists of the excited states. The excited states also interfere with the soliton, causing it to undergo temporal oscillations and a confined random walk within the central region of the halo. Using novel, high-resolution numerical simulations of an FDM halo corresponding to a particular boson mass, this dissertation demonstrates that the gravitational potential fluctuations associated with the soliton's random walk, its oscillations, and the envelope density fluctuations dynamically heat nuclear objects (e.g., central star clusters and supermassive black holes) and dwarf galaxies. As a result, nuclear objects, initially located at rest at the soliton center, migrate outwards over time until the outward motion is counteracted by dynamical friction, and an equilibrium is reached. Similarly, dwarf galaxies continue to increase their sizes and central velocity dispersions. In addition, their kinematic structures become strongly radially anisotropic, especially in the outskirts. Dynamical heating also causes initially ellipsoidal galaxies to become more spherical over time from the inside out and gives rise to distorted, non-concentric isodensity contours. Generalizing these results for other halo and boson masses and comparing them with observations (such as galaxy size-age relation, measured offsets of supermassive black holes and nuclear star clusters from the centers of their host galaxies) can potentially constrain the boson mass. In addition to studying the dynamics of collisionless particles in FDM, this dissertation also investigates the dynamics of the GC system in the recently discovered, dark matter deficient galaxy NGC 1052-DF2. By studying the dynamical friction-induced orbital decay of its GCs in different mass models that are allowed within the constraints from stellar and GC kinematics, it is demonstrated that a stars-only model (or a low mass dark matter core) is favored over a low mass dark matter cusp, as the presence of a dark matter cusp would lead to rapid orbital decay of the inner GCs, forming a nuclear cluster, which is not supported by observations. Finally, the probability of GC-GC mergers in this galaxy is also investigated, and it is shown that such events are rare and, therefore, cannot explain the existence of the overly luminous GCs.

• Publisher: ProQuest Dissertations & Theses
• Subject: Astronomy / Astrophysics
• ISBN: 9798368475752