Planck spots hot gas bridging galaxy cluster pair

ESA’s Planck 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. Planck’s main mission is a survey of the remnant light from the Big Bang and the Max Planck Institute for Astrophysics (MPA) in Garching has developed important software components for the mission and takes part in the analysis and scientific interpretation of the data.

Fig. 1: Planck has discovered a bridge of hot gas that connects the galaxy clusters Abell 399 (lower centre) and Abell 401 (top left). The galaxy pair is located about a billion light-years from Earth, and the gas bridge extends approximately 10 million light-years between them. The image shows the two galaxy clusters as seen at optical wavelengths with ground-based telescopes and through the Sunyaev-Zel'dovich effect (in orange) with the Planck satellite.
Credits: Sunyaev–Zel’dovich effect: ESA Planck Collaboration; optical image: STScI Digitized Sky Survey

Movie: This linkPfeil.gifmovie shows the three-dimensional structure of the cosmic web, the wispy network of both dark and baryonic matter that is believed to pervade the Universe. The movie was extracted from a numerical simulation of the formation and evolution of cosmic structure. Galaxy clusters are visible where the concentration of galaxies (shown in white and purple) is higher. Clusters form in the densest knots of the cosmic web, where filaments intersect. The density of gas in the filaments that link the cluster is represented with different colours, with dark brown indicating less dense regions and brighter colours (from orange to yellow and green) indicating increasingly denser regions. The colour scale also shows the gas temperature, ranging from less than a million K in the darker regions up to several tens of million K in the green regions.
Credit: Movie courtesy of Klaus Dolag, Universitäts-Sternwarte München, Ludwig-Maximilians-Universität München, Germany

Planck’s primary task is to capture the most ancient light of the cosmos, the Cosmic Microwave Background, or CMB. 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 CMB 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 (SZ) effect. This effect is named after the two scientists who predicted it in 1969: Rashid Sunyaev, presently director at the Max Planck Institute for Astrophysics, and Yakov Zel'dovich.

The SZ effect has already been used by Planck to detect linkPfeil.gifgalaxy clusters themselves, but it also provides a way to detect faint filaments of gas that might connect one cluster to another. Since the early universe, filaments of gaseous matter pervade the cosmos in a giant web, with clusters eventually forming in the densest nodes. Much of this tenuous, filamentary gas remains undetected, but astronomers expect that it could most likely be found between interacting galaxy clusters, where the filaments are compressed and heated up, making them easier to spot.

"Our numerical simulations of dark and baryonic matter agree pretty well with the statistical distribution of galaxies and galaxy clusters in the cosmic web," says Klaus Dolag, the scientists at the University Observatory Munich and the Max Planck Institute for Astrophysics, who provided the cosmological simulations for comparison to the observational data. "The challenge now is to try and also match the properties of the less dense filaments between the clusters, which are much harder to observe."

Planck’s discovery of a bridge of hot gas connecting the clusters Abell 399 and Abell 401, each containing hundreds of galaxies, represents one such opportunity. The presence of hot gas between the billion-light-year-distant clusters was first hinted at in X-ray data from ESA’s XMM-Newton, and the new Planck data confirm the observation.

“By combining Planck data with archival X-ray observations from the German satellite ROSAT, we estimated the temperature of gas in the bridge as about 80 million degrees Celsius,” notes Torsten Enßlin, a Planck Collaboration scientist from the Max Planck Institute for Astrophysics. Early analysis suggests the gas could be mixture of the elusive filaments of the cosmic web mixed with gas originating from the clusters. “It is still debated whether the gas derives from the intra-cluster medium or from the gas that was previously part of the two clusters; numerical simulations suggest that it could well be a mixture of both,” explains Enßlin. “Further analysis of the complete Planck data might help to clarify this issue by revealing additional examples,” he adds.

The new finding highlights the ability of Planck to probe galaxy clusters to their outskirts and beyond, examining their connection with the gas that permeates the entire Universe and from which all groups of galaxies formed.


linkPfeilExtern.gifESA Press Release

Original publication:

“Planck Intermediate Results. Hot diffuse gas between pairs of merging clusters as seen by Planck”, accepted for publication in Astronomy & Astrophysics.


Torsten Enßlin
MPA Planck Project Scientists
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Hannelore Hämmerle
Press Officer
Max Planck Institute for Astrophysics
and Max Planck Institute for extraterrestrial Physics
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Markus Bauer
ESA Science and Robotic Exploration Communication Officer
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