An emission map of the disk-circumgalactic medium transition in starburst IRAS 08339+6517

Most of a galaxy's mass is located beyond its stellar component, spread out to hundreds of kiloparsecs. This diffuse reservoir of gas, the circumgalactic medium, acts as the interface between a galaxy and the cosmic web that connects galaxies. We present kiloparsec-scale-resolution integral field spectroscopy of emission lines that trace cool ionized gas from the centre of a nearby galaxy to 30 kpc into its circumgalactic medium. We find a smooth surface brightness profile with a break in slope at twice the 90% stellar radius. The gas also changes from being photoionized by H II star-forming regions in the disk to being ionized by shocks or the extragalactic UV background at greater distances. This transition represents the boundary between the interstellar medium and the circumgalactic medium, revealing how the dominant reservoir of baryonic matter directly connects to its galaxy.