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There is no doubt that our atmosphere is an integral part of the ecosystem of the Earth. Everyday weather and long-term climate of the atmosphere are directly linked to activities on the surface of the Earth and vice versa. Gaseous halos, known as the circumgalactic medium (CGM), are the equivalent atmospheres of galaxies. The cycles of birth and death of stars are intricately tied to the cycle of gaseous structures in the galactic sky. This dissertation provides a major step towards characterizing and modeling the physical properties of the CGM and the co-evolution with their host galaxies. First, we statistically map the spatial extents of the CGM in ~200 galaxies that span a large range of stellar mass. This mapping shows a continuous detection of neutral hydrogen out to 10 virial radii of the host galaxies while heavy elements are detected only up to 50% - 70% of the virial radius. We explore the effects on the CGM profiles from different modeling of star formation and feedback processes using a set of high-resolution cosmological zoom-in simulations of a Milk-Way progenitor. This comparison illustrates that the properties of galaxies and the properties of their CGM provide strong complementary constraints on the processes governing galaxy formation. We show that the spatial profiles are self-similar across halo mass and cosmic time once it is re-normalized by the scaled radius of the host dark matter halo. To connect more closely with observations, we develop a synthetic absorption pipeline, as a virtual telescope, to observe the simulated galaxy and their CGM. Finally, using semi-analytic calculations, and a set of magnetohydrodynamic simulations, we present a multiphase model of the gaseous halos around galaxies, the circumgalactic mist. The model implies a large number of cold clumps in the inner halo with a small volume filling factor but a large covering fraction. The model naturally gives rise to spatial extents and differential covering fractions of cold, warm and hot gas. To self-consistently model the co-evolution of the CGM and star formation within galaxies, future simulations must address the mismatch of the spatial resolution and characteristic scale of cold gas.


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