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Scanning the sky in microwaves, the Planck mission has obtained its very first images of galaxy clusters, and found a previously unknown supercluster which is among one of the largest objects in the Universe. The supercluster is having an effect on the Cosmic Microwave Background, and the observed distortions of the CMB spectrum are used to detect the density perturbations of the universe, using what is called the Sunyaev–Zel’dovich effect (SZE). This is the first time that a supercluster has been discovered using the SZE. In a collaborative effort, the XMM Newton spacecraft has confirmed the find in X-rays.
Sunyaev-Zel’dovich Effect (SZE) effect describes the change of energy experienced by CMB photons when they encounter a galaxy cluster as they travel towards us, in the process imprinting a distinctive signature on the CMB itself. The SZE represents a unique tool to detect galaxy clusters, even at high redshift. Planck is able to look across nine different microwave frequencies (from 30 to 857 GHz) to remove all sources of contamination from the CMB, and over time, will provide what is hoped to be the sharpest image of the early Universe ever.
“As the fossil photons from the Big Bang cross the Universe, they interact with the matter that they encounter: when travelling through a galaxy cluster, for example, the CMB photons scatter off free electrons present in the hot gas that fills the cluster,” said Nabila Aghanim of the Institut d’Astrophysique Spatiale in Orsay, France, a leading member of the group of Planck scientists investigating SZE clusters and secondary anisotropies. “These collisions redistribute the frequencies of photons in a particular way that enables us to isolate the intervening cluster from the CMB signal.”
Since the hot electrons in the cluster are much more energetic than the CMB photons, interactions between the two typically result in the photons being scattered to higher energies. This means that, when looking at the CMB in the direction of a galaxy cluster, a deficit of low-energy photons and a surplus of more energetic ones is observed.
The SZE signal from the newly discovered supercluster arises from the sum of the signal from the three individual clusters, with a possible additional contribution from an inter-cluster filamentary structure. This provides important clues about the distribution of gas on very large scales which is, in turn, crucial also for tracing the underlying distribution of dark matter.
“The XMM-Newton observations have shown that one of the candidate clusters is in fact a supercluster composed of at least three individual, massive clusters of galaxies, which Planck alone could not have resolved,” said Monique Arnaud, who leads the Planck group following up sources with XMM-Newton.
“This is the first time that a supercluster has been discovered via the SZE,” said Aghanim. “This important discovery opens a brand new window on superclusters, one which complements the observations of the individual galaxies therein.”
Superclusters are large assemblies of galaxy groups and clusters, located at the intersections of sheets and filaments in the wispy cosmic web. As clusters and superclusters trace the distribution of both luminous and dark matter throughout the Universe, their observation is crucial to probe how cosmic structures formed and evolved.
The first Planck all-sky survey began in mid-August 2009 and was completed in June 2010. Planck will continue to gather data until the end of 2011, during which time it will complete over four all-sky scans.
The Planck team is currently analyzing the data from the first all-sky survey to identify both known and new galaxy clusters for the early Sunyaev-Zel’dovich catalogue, which will be released in January of 2011.