For the first time, ESA's Planck space telescope has detected a bridge of hot gas connecting a pair of galaxy clusters 10 million light-years apart.
Abell 399 and Abell 401, each containing hundreds of galaxies, are billions of light years away. The presence of hot gas linking them had already been hinted at in X-ray data from ESA's XMM-Newton, but is now confirmed.
"Planck is helping to reveal hidden material between galaxy clusters that we couldn't see clearly before," says James Bartlett of NASA's Jet Propulsion Laboratory.
In the early universe, filaments of gaseous matter pervaded the cosmos in a giant web, with clusters eventually forming in the densest nodes. The gas that remains is easiest to spot between interacting galaxy clusters, where the filaments are compressed and heated up.
The discovery marks Planck’s first detection of inter-cluster gas using the SZ effect technique.
If the cosmic microwave background light interacts with the hot gas permeating these huge cosmic structures, its energy distribution is modified in a characteristic way, a phenomenon known as the Sunyaev-Zel'dovich effect, after the scientists who discovered it.
By combining Planck data with archival X-ray observations from the German satellite Rosat, the scientists concluded that temperature of the gas in the bridge is to be similar to the temperature of the gas in the two clusters – roiughly 80 million degrees Celsius.
Early analysis suggests that it could be a mixture of the elusive filaments of the cosmic web mixed with gas originating from the clusters.
A more detailed analysis - and maybe the discovery of gas bridges connecting other clusters - will help provide a more conclusive answer.