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November 20, 2012


The 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 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.

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.

Astronomers using Planck and the Sunyaev-Zel'dovich effect were able to discover a bridge of hot gas connecting the clusters Abell 399 and Abell 401, each containing hundreds of galaxies.

The presence of hot gas between the clusters, which are billions of light years away, was first hinted at in X-ray data from ESA's XMM-Newton, and the new Planck data confirm the observation.

  Intergalactic Gas Bridge Seen by Planck Credits: ESA Planck Collaboration (Sunyaev-Zel'dovich effect, in orange); STScI Digitized Sky Survey (optical image)   Intergalactic Gas Bridge Seen by Planck

This image shows the two galaxy clusters Abell 399 and Abell 401, which can be seen in the upper left corner (Abell 401) and in the central lower portion of the image (Abell 399), respectively. They are located at redshift z~0.07 – about one billion light-years away from us.

In the optical image, obtained with ground-based telescopes, galaxies are visible as slightly elliptical, bright flecks. The two clusters can be seen as the two portions of the images that show a higher concentration of galaxies. The image is also dotted with several foreground stars that belong to our Galaxy, the Milky Way.

Depicted in orange is the hot gas that pervades the two clusters and the space between them, based on data from ESA's Planck satellite. Planck can detect the hot gas via the Sunyaev-Zel'dovich effect.

In addition to a significant number of galaxies, galaxy clusters contain hot gas and large amounts of dark matter. The hot gas in galaxy clusters manifests itself in two ways: directly, through its X-ray emission, and indirectly, via a characteristic signature it imprints on the Cosmic Microwave Background, known as the Sunyaev-Zel'dovich effect. By surveying the sky at microwave and sub-millimetre wavelengths, Planck is an ideal tool to hunt galaxy clusters across the sky exploiting the Sunyaev-Zel'dovich effect.

Astronomers analysing the Planck data for Abell 399 and Abell 401 have not only detected the individual signal from the gas in each cluster (which can be seen in the image as the two, bright orange regions) but also a 'bridge' of gas linking the two (the dimmer orange filament connecting the two brighter orange regions). The filament extends over about 10 million light-years and contains gas with a temperature of about 80 million K. At least part of this inter-cluster gas might derive from the warm-hot intergalactic medium (WHIM) – the elusive web of gaseous filaments which is believed to pervade the Universe.

Read the full story from the European Space Agency at

NASA's Planck Project Office is based at JPL. JPL contributed mission-enabling technology for both of Planck's science instruments. European, Canadian and U.S. Planck scientists work together to analyze the Planck data. More information is online at and

Whitney Clavin 818-354-4673
Jet Propulsion Laboratory, Pasadena, Calif.


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