The Circumgalactic Medium

Abstract: Galaxies are part of a vast cosmic ecosystem, embedded in an extensive gaseous reservoir that regulates their growth by providing the necessary fuel for star formation while preserving a fossil record of past interactions, outflows, and feedback processes. The circumgalactic medium (CGM) contains multiphase gas spanning a broad dynamic range in spatial scale, density, and temperature, with its thermodynamic and chemical properties deeply linked to the star formation histories of galaxies. As a rich laboratory for studying gas physics, the CGM offers unique insights into the processes governing gas cooling, heating, and material transfer between galaxies and their surroundings. Chemical tagging, based on the relative abundances of multiple elements, serves as a powerful timing tool to trace the origin of the gas and connect the stars in the interstellar medium (ISM) to the diffuse CGM. Developing a complete understanding of the CGM and its cosmic evolution requires multi-wavelength observational tools, ranging from X-ray, UV/optical, and sub-mm to radio, to probe this diffuse gas in both absorption and emission.