Physics Question #3583

Ernie Epp, a 76 year old male from Kenora, ON asks on August 31, 2006,

Collision of galaxie clusters. In recent news reports that I've read it has been reported that further proof of dark matter has been established. At the same time the story says that the collision has produced extremely high temperature gases of something like 20 million degrees (I've forgotten the exact figure) . Why is this product referred to as a gas. I had thought that any substance of this high temp has been reduced to its ultimate primary form such as neutrons, protons, quarks etc. etc. Don't you think that this high temp substance should have a new name? What actually is it at those extremely high temperatures?

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The answer

Leigh Palmer, Physics professor, Simon Frasser University, Burnaby, BC Canada answered on October 4, 2006

This state of matter is usually called a plasma rather than a gas, but astronomy and astrophysics are very conservative disciplines that tend to preserve traditional terms. (If you want an extreme example of this look up "galactic clusters". Now that's a confusing name!) When it was first discovered in rich clusters of galaxies this plasma was called "cluster gas" or "intracluster gas" and the name has stuck. This adherence to tradition has served astronomers well because language doesn't change capriciously over time, and early literature is easily accessible to modern readers without the necessity of translation. We've known of the existence of this particular form of dark matter for many years. This paper purports to demonstrate that the cluster gas is more massive than the visible galaxies themselves.

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