What state of gas characterizes the enormous halos that envelop many large elliptical galaxies?

Answer

Hot gas detected through X-ray observations

Many large elliptical galaxies are enveloped by enormous halos of hot gas, which can be detected because it reaches millions of degrees and emits X-rays.

What state of gas characterizes the enormous halos that envelop many large elliptical galaxies?
galaxiesastronomygasdust