ASK SPACE ASTROPHYSICS

Many galaxies in the universe live alongside others, tethered to each other by their mutual gravity, to form giant clusters; the largest gravitationally bound structures in the universe. The galaxies in a cluster are only a tiny fraction of the total matter present – there’s up to ten times as much mass in the form of a hot tenuous atmosphere filling the space between the galaxies, known as the intracluster medium.

The enormous gravitational field of the whole cluster – particularly that of the dominant but unobservable dark matter – squeezes this gas to heat it to extraordinarily high temperatures of millions of degrees. There is so much energy in this gas that the electrons are no longer bound to their atomic nucleus, but instead form a plasma composed completely of electrically charged particles. Too hot to be seen in optical images, this plasma is only observable in the X-ray waveband where it appears as a giant, smooth puddle centred on the most massive galaxy in the cluster.

The temperature of the intracluster medium is a puzzle – it radiates so much energy through its X-ray emission that it should have long since cooled down. The fact that it remains so hot requires a continual heating mechanism within the cluster, most likely related to the mechanical energy of sound waves travelling through the cluster gas. These waves in turn originate from disturbances caused by the active supermassive black hole found at the core of the central and most massive galaxy.