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The circumgalactic medium (CGM) is a vital element in galaxies, as it mediates the baryon cycle essential for regulating galaxy activity. It is also highly complex due to the intricate distributions of temperature, density, metallicity, and ionization that make the CGM a multiphase medium. Therefore, learning about the CGM requires combining various observational techniques. This contribution starts by reviewing how absorption spectroscopy, together with modeling of the ionization conditions, yields critical insights into the underlying physical properties of the CGM. Next, the rapidly growing application of imaging and integral field spectroscopy for studying the halo gas in emission, using hydrogen and metal lines as tracers, is examined. Finally, the essential role of the CGM in galaxy evolution is highlighted by considering current studies that directly link galaxies to their halo gas. The novel dimension of how the environment affects the CGM and alters the evolution of galaxies is also investigated.
Michele Fumagalli (Fri,) studied this question.