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MPE Press Release May 31, 2010 |
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Novel observing mode on XMM-Newton opens new perspectives on galaxy clustersSurveying the sky with XMM-Newton, scientists at the Max-Planck-Institute for Extraterrestrial Physics and other institutes have discovered two massive galaxy clusters, confirming a previous detection obtained through observations of the Sunyaev-Zel'dovich effect, the "shadow" they cast on the Cosmic Microwave Background. The discovery, made possible thanks to a novel mosaic observing mode recently introduced on ESA's X-ray observatory, opens a new window to study the Universe's largest bound structures in a multi-wavelength approach.
Galaxy clusters are the largest gravitationally bound objects in the Universe. As such, they are extremely important probes of cosmic properties on very large scales, since they form in the densest knots of the large-scale structure, the cosmic web. Originally discovered as an excess density (or cluster) of galaxies located at the same redshift, hence the name, there is much more to these enormous structures than mere galaxies: in fact, only about one tenth of the entire mass of a galaxy cluster arises from galaxies (up to a thousand in the most massive cases), another tenth consists of hot gas, and the remainder can be attributed to dark matter. The gas that fills galaxy clusters is hot enough to emit X-rays - with a temperature of more than 10 million Kelvin, the gas is ionised and electrons scattering off ions are decelerated, emitting radiation in the process. From measurements of the X-ray luminosity of galaxy clusters and of the gas temperature, the total mass of these structures can be estimated. This yields clear evidence that clusters are indeed gravitationally bound structures and that their mass is dominated by the elusive and invisible dark matter. "Interestingly, the same hot gas we directly observe in X-rays also affects the photons of the Cosmic Microwave Background (CMB), which are passing through the cluster on their way to us," says Hans Böhringer from the Max Planck Institute for Extraterrestrial Physics. The CMB photons interact with the extremely energetic electrons in the cluster plasma and in doing so their energy is modified in a very characteristic way, leaving a signature on the CMB - the so-called Sunyaev-Zel'dovich Effect (SZE). "We can then see clusters as 'shadows' cast on the CMB in the millimetre subset of radio wavelengths," Böhringer adds. A survey of the sky at millimetre wavelengths, currently being carried out with the South Pole Telescope (SPT), has recently achieved its first results, detecting a dozen of previously unknown galaxy clusters by means of their SZE signature. Follow-up observations in the optical and X-rays are, however, needed in order to better characterise the physical properties of these structures and to probe how the observed SZE signal depends on the mass of the clusters.
The discovery was possible thanks to a new mode of observations recently implemented by the XMM-Newton Science Operations Centre. "The new mosaic observing mode enables us to survey large areas of the sky in a much more efficient way than previously," explains Maria Santos-Lleo, XMM-Newton Science Support Manager. ESA's X-ray observatory has been operating for more than ten years, but the demand for observing time is still high and is often driven by new science goals - some of them unexpected during the project phase, over a decade ago. In some cases, the scientific objectives require the observation of sky regions larger than the field of view of the cameras aboard the spacecraft. This pushes the support scientists to implement new operating modes that optimise the performance of the instruments. "It is difficult, and very rare, to develop new modes when the spacecraft is already in orbit and operating. In this particular case, we succeeded in figuring out a novel way to exploit the instruments in order to satisfy new needs of the astronomical community," adds Santos-Lleo. Thanks to the mosaic mode, it was possible to extend the observed patch of the sky to about 14 square degrees, about 70 times the area of the full Moon. Besides the SZE detection and X-ray data, optical observations of the galaxies in the two clusters enabled their redshifts to be established: z=0.3 (in the case of the more massive one) and z=1.0, respectively. This is the very first joint discovery of galaxy clusters in a sky survey combining data probing these three different wavebands. "This survey not only shows that we can efficiently detect galaxy clusters in all these wavelengths, but also that the cluster redshifts reach easily as far as z=1, a necessary condition to follow structure evolution over an interesting cosmological time span," Hans Böhringer comments. The most distant of the two clusters is in fact seen as it was when the Universe was barely 6000 million years old, less than a half of its current age.
Original paper: XMM-Newton detection of two clusters of galaxies with strong SPT Sunyaev-Zel'dovich effect signatures R. Šuhada, J. Song, H. Böhringer, B. A. Benson, J. Mohr, R. Fassbender, A. Finoguenov, D. Pierini, G. W. Pratt, K. Andersson, R. Armstrong and S. Desai 
Astronomy & Astrophysics Letters 514, L3 (2010)
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Last update: 2010-05-31 by
 H. SteinleContact: MPE public outreach department |
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