circumgalactic medium
/ ser-kum-guh-LAK-tik /
A galaxy is not just the bright pinwheel of stars you see in pictures. Surrounding that visible disk is a huge, faint halo of gas extending far beyond the stars — an atmosphere for the whole galaxy. The circumgalactic medium, or CGM, is this reservoir of gas that envelops a galaxy, sitting in the borderland between the galaxy itself and the truly intergalactic space beyond.
The CGM is the diffuse gas filling a galaxy's halo, reaching out hundreds of thousands of light-years — often larger than the visible galaxy itself. It is a turbulent, multi-temperature mixture: cool clouds falling inward, hot gas blown out by supernovae and galactic winds, and gas slowly cycling between the two. Crucially, it can hold as much ordinary matter as all the galaxy's stars combined, perhaps more. Because it is so faint, the CGM is studied mostly in silhouette: astronomers look at the light of a bright background quasar that happens to shine through a foreground galaxy's halo, and read the absorption lines the halo gas imprints on that light.
The circumgalactic medium is the lungs of a galaxy. Fresh gas flows in from the cosmic web through the CGM to fuel new stars, while feedback from supernovae and black holes drives enriched gas back out into it — a constant breathing in and out. How well a galaxy can pull gas from its CGM may decide whether it keeps making stars or 'quenches' and goes quiet. Once an afterthought, the CGM is now seen as central to understanding why galaxies look the way they do, holding the gas budget that the bright, starry part merely draws upon.
When the light of a distant quasar happens to pass through the halo of a nearer galaxy, the galaxy's circumgalactic gas stamps absorption lines onto the quasar's spectrum — letting astronomers weigh and map gas they could never see glowing on its own.
Background quasars probe the invisible halo gas in absorption.
The circumgalactic medium is the gaseous halo immediately around a galaxy — distinct from the truly intergalactic medium far between galaxies, though the two blend into each other with no sharp boundary. The CGM is a galaxy's near gas, the IGM the great void beyond.