Circumgalactic medium
The circumgalactic medium is the gas that sits around a galaxy, outside the disk but inside the halo that holds it. It is where galaxies get their fuel: it feeds star formation, it catches the material blown back out by winds and jets, and it regulates how much gas a galaxy has to work with. In the early universe this reservoir is cool and dense enough to glow in the Lyman alpha line, which makes it possible to see directly, as an extended halo of emission around a bright quasar.
What is not settled is what the quasar’s own jets do to that gas. Radio-loud quasars drive powerful relativistic jets straight into their surroundings, and whether that disrupts the reservoir, compresses it, or barely touches it is an open question at these redshifts.
