Q&A: Galaxies, Galaxy Clusters, AGN, and Quasars

Q:
How can gases maintain temperatures of millions of degrees in the "void" of interstellar and intergalactic space for extended periods of time?

A:
The gas in galaxy clusters is hot because they have collapsed, or fallen inward. For example, if you drop a book on the floor, what happens? It accelerates under gravity, and picks up energy of motion until it hits the floor, at which point it releases energy as sound. When a cluster collapses, the gas particles collide and the energy of collapse is converted to heat. The same thing happens with galaxies and stars but they get rid of the heat by radiating it away as light in a few million years. Clusters are of great interest to astronomers because they are the largest gravitationally bound structures in the universe, and because the gas is so spread out that it retains the heat from the collapse for billions of years. The history of the collapse, and the effects of galactic explosions, which can also add heat to the gas, is there for us to read, using the X-rays as the alphabet.