Planck discovers filament of hot gas linking two galaxy clusters

Modern cosmological theories state that the universe is mainly composed of dark matter and dark energy. Baryonic matter makes up the stars and planets. In this form it represents only 5% of the total matter content in the universe. This material is also present in the form of hot gas between the galaxies. However, to date, astronomers have identified about half of the baryonic matter present in the local universe.

Numerical simulations of the forming of large cosmic structures (galaxies and galaxy clusters) suggest that the other half of the baryonic matter would be in the form of a warm gas, called WHIM (Warm Hot Intergalactic Medium) distributed within the cosmic web, the filament network that  structures of dark and baryonic matters form in the Universe. Although its temperature ranges from 100,000 to several million degrees, the gas, due to its very low density, is particularly difficult to detect.

The Planck mission has discovered a filament of hot gas connecting two galaxy clusters named Abell 399 and Abell 401.

The hypothesis that the missing baryonic matter is to be found in the  intergalactic hot gas was confirmed by observations in the X, UV and optical domains. It is now reinforced by the treatment of data coming from the ESA’s Planck satellite in the fields of microwaves and sub-millimeter.

The primary mission of the Planck satellite is to detect inhomogeneities in the CMB. Some of these inhomogeneities result from the interaction between the CMB and the hot gas contained in galaxy clusters, a phenomenon known as the Sunyaev-Zel’dovich effect. The exploitation of this effect at first allowed the researchers of the Planck collaboration to detect  galaxy clusters then to draw up a catalog of about 200 clusters . The next step consisted in observing the regions located at the intersection between pairs of galaxy clusters relatively close one to another in the local Universe. In principle, the gravitational interaction between clusters should result in an increase in the temperature and density of the inter-cluster gas, and thus make it easier to detect. Indeed, the examination of pairs of candidate clusters revealed the existence of a “material bridge” between the clusters Abell 399 and Abell 401 located approximately one billion light-years from Earth. This filament of hot gas extends over ten million light-years.

This is the very first detection of the inter-cluster gas obtained by Sunyaev-Zel’dovich effect. Combined analysis of Planck data with those of the ROSAT satellite in the X domain was used to estimate the temperature of the gas to about 80 million degrees, a temperature close to that of the intracluster gas. The origin of this gas remains uncertain, it is indeed difficult to establish whether it comes from the gas making up each cluster, or from the gas more widely distributed in the cosmic web, or from a combination of both gases. The detailed analysis of other pairs of clusters detected by the Planck satellite will undoubtedly refine the answer to this question.

For the detail : The IRAP’ Planck team coordinate studies upon galaxy clusters via the measurement of the Sunyaev-Zel’dovich effect in the Planck data. They are heavily involved in the characterization of the properties of the hot gas bathing the large structures of the Universe.

Further details :

IRAP Contact :

  • Etienne Pointecouteau,

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