Figuring Out Gas & Galaxies In Enzo (FOGGIE). XIV. The Observability of Emission from Accretion and Feedback in the Circumgalactic Medium with Current and Future Instruments

Abstract Observing the circumgalactic medium (CGM) in emission lines from ionized gas enables direct mapping of its spatial and kinematic structure, offering new insight into the gas flows that regulate galaxy evolution. Using the high-resolution Figuring Out Gas & Galaxies In Enzo (FOGGIE) simulations, we generate mock emission-line maps for six Milky Way–mass halos. Different lines (e.g., H α , O vi ) trace distinct CGM phases and structures, highlighting the need for multispecies observations. We quantify the observable CGM mass fraction as a function of instrument spatial resolution and surface brightness sensitivity, finding that sensitivity is the dominant factor limiting detectability across all ions. At fixed sensitivity, higher spatial resolution reveals more structures; at fixed spatial resolution, higher sensitivity recovers a higher percentage of the total mass. We construct emissivity-weighted projected velocity maps and compare line-of-sight velocities between emission lines. O vi shows the largest kinematic deviation from H α , while Mg ii and Si ii most closely follow H α velocities. Distinguishing these phases out to 50 kpc from the galaxy center requires spectral resolution better than 30 km s −1 for most ion pairs. Separating inflowing from outflowing gas based on projected kinematics requires high spectral resolution: at 30 km s −1 , more than 80% of gas above the emission detection threshold can be distinguished kinematically, but this fraction drops to < 40% with a resolution of 200 km s −1 . Our results provide predictions for future instruments, showing that recovering the multiphase structure and kinematics of the CGM in emission will require both high sensitivity and fine kinematic resolution.