The circumgalactic medium (CGM) plays a central role in galaxy evolution, acting as both the reservoir for gas accretion and the interface through which feedback regulates star formation. However, direct observational constraints on the spatial structure and physical conditions of the CGM remain limited. The low-redshift universe provides a unique opportunity in which host galaxy properties can be well constrained, and inflows and outflows can be resolved with sufficient spatial detail to directly probe the interaction between the CGM and the interstellar medium (ISM). In this thesis, I present a study of the ionized CGM using deep integral-field spectroscopy with the Keck Cosmic Web Imager (KCWI), based on a survey of 12 star-forming galaxies spanning nearly two orders of magnitude in stellar mass at z ~ 0. 1. These observations enable sensitive mapping of faint emission lines such as H-alpha, O III 5007, and O II 3727, 3729, yielding spatially resolved measurements of CGM emission on scales of tens of kiloparsecs and opening a new window into the kinematics and ionization structure of diffuse halo gas. A detailed case study of the low-mass galaxy J0910b reveals extended CGM emission with complex kinematics, including evidence for filamentary inflow and counter-rotation relative to the host galaxy. Emission-line ratios indicate that the gas is predominantly photoionized, consistent with a diffuse ultraviolet (UV) background and leakage of ionizing photons from star formation, rather than strong shock heating. Extending this analysis to a broader sample, I identify a coherent scaling relation between CGM emission properties and galaxy stellar mass, defining a “CGM main sequence. ” Across the sample, the radial extent and surface brightness of emission-line halos exhibit self-similar behavior when scaled by virial radius, with a clear transition near log (Mₛtar/Mₛun) ~ 10. 5. This sequence indicates that the cool ionized CGM traces the underlying hot, diffuse halo and is linked to the dark matter potential. In this framework, a galaxy’s position reflects the balance between gas accretion, star formation, and feedback over its assembly history: low-mass, actively star-forming galaxies occupy a bright, cool-gas-rich branch capable of sustaining continued growth, whereas higher-mass systems lie on a suppressed, baryon-poor branch shaped by strong feedback and virial heating as they transition toward quenching. This result establishes the first structural scaling law for the ionized CGM in individual low-redshift galaxies, in direct analogy to the star-formation main sequence. Optical observations primarily probe the cool (~10⁴ K) phase of the CGM. Key tracers of the warm-hot phase, such as Ly-alpha, C IV 1548, 1550, and O VI 1032, 1038, are shifted into the UV. To access this regime, our group has led the FIREBall-2 (Faint Intergalactic-medium Redshifted Emission Balloon) experiment, a suborbital UV spectroscopic mission designed to probe the multiphase structure of the CGM. I contributed to the development and optimization of the FIREBall-2 UV detector system, including performance characterization of a photon-counting EMCCD during the 2022–2023 flight campaigns. Although no UV science data were obtained due to a balloon envelope leak, extensive pre-flight testing demonstrated the instrument performance and advanced its technology readiness level, paving the way for future UV missions probing the full multiphase CGM.
Zeren Lin (Wed,) studied this question.