UC Riverside

In this dissertation, we explore the possibility of using a novel dark matter model, i.e., self-interacting dark matter (SIDM), to address the challenges that the traditional cold dark matter (CDM) paradigm has in explaining observations on galactic scales. For example, in spiral galaxies, galactic rotation curves exhibit a diverse behavior in the inner regions, while obeying an organizing principle, i.e., they can be approximately described by a radial acceleration relation. However, current CDM simulations can not offer a satisfactory explanation for both of them simultaneously. Additionally, in early-type galaxies (ETGs), the smallness of dark matter fractions within their effective radii indicates no signs of adiabatic contraction processes predicted by CDM simulations, and the nearly isothermal total density profiles featured with small scatters within large radii are also beyond naive expectations.

We apply the SIDM model to study kinematics and dynamics of these two main types of galactic systems. For spiral galaxies, we analyze the rotation curve data from the SPARC sample, and explicitly demonstrate that both the diversity and uniformity are naturally reproduced in a hierarchical structure formation model with the addition of dark matter self-interactions. Meanwhile, other properties such as the concentrations of dark matter halos, the stellar mass-to-light ratio distribution etc. are well consistent with observations and theoretical considerations. We also compare the SIDM results with hydrodynamical CDM simulations with strong baryonic feedback, and highlight the importance of the diverse baryon profiles in fitting to the rotation curves. For ETGs, we model the SIDM halos and baryon profiles with data from the SL2S sample. The total density profiles are close to the isothermal style and the inferred dark matter fractions match well with the data derived from strong lensing and stellar kinematics.

We show that SIDM can provide an economical explanation to the observations of spiral galaxies and ETGs, while being consistent with CDM predictions on large scales. Our results provide compelling arguments in favor of the idea that the inner halos of galaxies are thermalized due to dark matter self-interactions.