Astronomers have discovered a giant wave of gas, about twice the size of the Milky Way, making its way through the Perseus galaxy cluster. Hot gas is a common component of intergalactic space inside a cluster but the wave is definitely something more peculiar, forming an unusual concave feature called the bay.

Astronomers believed the configuration was a consequence of the supermassive black hole inside NGC 1257, the central galaxy in the cluster. But detailed X-ray analysis, performed with NASA’s Chandra, exonerates the supermassive black hole. The bay surrounding NGC 1257 had no X-ray emissions, the opposite of what astronomers would expect.

As reported in the Monthly Notices of the Royal Astronomical Society, the wave was likely formed by a cluster merger. Scientists believe that a smaller cluster missed the central region by about 650,000 light-years, but the passage generated these incredible waves in the gas.

^{Visible and enhanced X-ray view of the Perseus Cluster. NASA/CXC/SAO/E.Bulbul, et al. and NASA's Goddard Space Flight Center/Stephen Walker et al.}

"Perseus is one of the most massive nearby clusters and the brightest one in X-rays, so Chandra data provides us with unparalleled detail," said lead author Stephen Walker, from NASA's Goddard Space Flight Center, in a statement. "The wave we've identified is associated with the flyby of a smaller cluster, which shows that the merger activity that produced these giant structures is still ongoing."

The solution to the mystery came from a sophisticated computer simulation that tried to recreate the dynamic environment of the cluster. The gas had previously set into a “cold” central region at 30 million °C (54 million °F) surrounded by a region three times as hot.

Then the merger happened and all that gas got mixed together forming the wave, which is believed to be a Kelvin-Helmholtz wave, like the waves you get on the surface of the sea or in clouds.

"We think the bay feature we see in Perseus is part of a Kelvin-Helmholtz wave, perhaps the largest one yet identified, that formed in much the same way as the simulation shows," Walker said.

Researchers have recently shown similar features are present in two other galaxy clusters, Centaurus and Abell 1795.

Those two clusters and the Perseus cluster, also known as Abell 426, are some of the most massive objects in the known universe, with hundreds of times the mass of the Milky Way contained in their central region. Studying their turbulent gas might open a door into their still unclear past.