Explore the Nature of Dark Matter in the Context of Galaxy Formation

Citation

Shen, Xuejian (2023) Explore the Nature of Dark Matter in the Context of Galaxy Formation. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/zccb-sr51. https://resolver.caltech.edu/CaltechTHESIS:05252023-181654402

Abstract

The nature of dark matter (DM) is a fundamental question in modern cosmology. Despite its significant role in various physical processes throughout the Universe, the particle nature of DM remains elusive. With the non-detection of classical candidates (e.g. WIMPs), the theoretical space for DM is becoming increasingly open. This thesis revolves around studying the nature of DM in the context of structure formation and we will focus on a category of DM with self-interactions (SIDM), which can be constrained only through astrophysical probes if DM has no coupling with the standard model particles. Utilizing advanced cosmological hydrodynamical simulations, we examine the effects of DM elastic and dissipative self-interactions on galaxy structure and their interplay with baryonic physics processes. Our numerical studies encompass a range of systems, such as Local dwarf galaxies, massive galaxy clusters in the Local Universe, and rare massive quasar-host galaxies at high redshift (z ≳ 6). In Local dwarf galaxies, we analyze the unique signatures of dissipative self-interacting DM (dSIDM) with typical self-interaction cross-section σ/m ~ 0.1-10 cm² g⁻¹ and dissipation factor ~ 0.5. We find a universal cuspy central density profile and systematic changes in halo morphology in dSIDM. By comparing our results with observations, we derive constraints for effective parameters of dSIDM and identify the parameter space where it remains viable and exhibits interesting observational implications. For a similar type of dSIDM with fairly low σ/m ≾ 0.05 cm² g⁻¹, we also explore the possibility that the direct collapse of dSIDM halos at high redshift can seed supermassive black holes and serve as progenitors for massive bright quasars observed at high redshift. This scenario predicts a large population of quiescent supermassive black holes (SMBHs) at high redshift, which could be tested by future LISA observations. Lastly, in Local massive galaxy clusters, we compare the X-ray morphology of hot gas in observed clusters with simulations of elastic SIDM. Although SIDM models with large interaction cross-sections (σ/m ≳ 0.5 cm² g⁻¹) are favored, uncertainties from cooling and feedback physics in galaxy clusters must be taken into account. This thesis summarizes the findings and constraints on DM properties, with a particular emphasis on its potential self-interactions, as derived from a combination of research projects.

Thesis Files