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

Q:
I'm mystified by the announcement that the Chandra X-Ray Observatory has imaged sounds waves in Perseus. What medium is modulated? While the waves are reported to be very low frequency, the implication is that there are areas of compression and rarefaction. Of what? The "ether" of space?

A:
There is hot gas in clusters of galaxies (detectable with X-ray telescopes), and this is the medium that the sound travels through. The density of this gas is very low, but as long as there is some continuous medium, then sound is able to pass through it.