Colossal Galactic Bridge
The Planck (European Space Agency or ESA) space telescope has made the first conclusive detection of a bridge of hot gas connecting a pair of galaxy clusters across 10 million light-years of intergalactic space. In the early Universe, filaments of gaseous matter pervaded the cosmos in a giant web, with clusters eventually forming in the densest nodes. Planck’s discovery of a bridge of hot gas connecting the clusters Abell 399 and Abell 401, each containing hundreds of galaxies is one such discovery.
"Planck is helping to reveal hidden material between galaxy clusters that we couldn't see clearly before," said James Bartlett of NASA's Jet Propulsion Laboratory, Pasadena, Calif., a member of the U.S. Planck science team. Planck is a European Space Agency mission with significant participation from NASA.
Galaxy groups and clusters are the largest known gravitationally bound objects to have arisen thus far in the process of cosmic structure formation. They form the densest part of the large scale structure of the Universe. In models for the gravitational formation of structure with cold dark matter, the smallest structures collapse first and eventually build the largest structures, clusters of galaxies. Clusters are then formed relatively recently between 10 billion years ago and now. Groups and clusters may contain ten to thousands of individual galaxies.
The mission's primary task is to capture the most ancient light of the cosmos, the cosmic microwave background. As this faint light traverses the universe, it encounters different types of structure, including galaxies and galaxy clusters -- assemblies of hundreds to thousands of galaxies bound together by gravity.
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. Essentially, the hot gas molecules in the cluster scatter cosmic microwave background photons up to higher energies. Thus, a "hole" or shadow appears in an image made with microwave light.
By combining the Planck data with archival X-ray observations from the German satellite Rosat, the temperature of the gas in the bridge is found to be similar to the temperature of the gas in the two clusters — on the order of 80 million degrees Celsius.
For further information see Bridge.
Abell image via NASA.