They must be turning infrared intensity at various frequencies to (r,g,b) in a way that longer wavelengths contribute to the r channel and shorter wavelengths to the b channel because they say "more distant galaxies" look redder. See

https://ned.ipac.caltech.edu/level5/March02/Sarazin/frames.h...

for some explanation of how the gas gets so hot

This garden universe ... vibrates complete. Some may get a sound so sweet. Vibrations reach on up to become light, and then through gamma, out of sight. Between the eyes and ears there lie the sounds of color and the light of a sigh. And to hear the sun, what a thing to believe ... but it's all around if we could but perceive.

To know ultra-violet, infra-red, and x-rays ... beauty to find in so many ways. Two notes of the chord, that's our full scope, but to reach the chord is our life's hope. And to name the chord is important to some. So they give it a word, and the word is "OM".

https://www.youtube.com/watch?v=N36oRaEF4jA