Observing the circumgalactic medium in emission would provide us with a 3d map of the spatial and kinematic extent of the gas fueling and impacted by galaxy evolution. Emission brightness, however, is proportional to density squared: the diffuse CGM is very difficult to observe in emission. As advances in ground-based instrumentation have begun to push the envelope of what is observable, it has become increasingly important to understand how obsrvations of circumgalactic emission relate back to the underlying physical conditions of the gas.
Corlies et al. (2018) "Figuring Out Gas & Galaxies in Enzo (FOGGIE). II. Emission from the High-Redshift Circumgalactic Medium", arXiv:1811.05060, ApJ submitted