The ESA's Planck observatory finds a connection between Abell 399 and Abell 401.
About a billion light years away from us here on Earth, there are two galaxy clusters, Abell 399 and Abell 401. Those clusters are distant, and not just from us, but from each other: They are located some 10 million light years apart. But those clusters are also, it turns out, connected -- through a bridge, composed of hot gas, that conjoins them across space. Planck, the European Space Agency space observatory, made the discovery.
Shown above are Abell 399 and Abell 401, as seen at optical wavelengths through ground-based telescopes -- and as observed by Planck. The orange coloring indicates the presence of the Sunyaev-Zel'dovich effect, which causes a change in the apparent brightness of the cosmic microwave background radiation between stars and galaxies (and, for that matter, between any other reservoirs of hot plasma out there in the universe).
The bridge Planck discovered could be evidence that the galaxy clusters in question passed by each other many, many ages ago. That was the going theory, at least, when a similar gas bridge -- a stream of hydrogen -- was discovered this summer, this one connecting the colossal Andromeda Galaxy, M31, and its neighboring Triangulum Galaxy, M33. "The properties of this gas indicate that these two galaxies may have passed close together in the distant past," Jay Lockman, of the National Radio Astronomy Observatory, said of that discovery. "Studying what may be a gaseous link between the two can give us a new key to understanding the evolution of both galaxies."