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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 clusters

Surveying 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 cluster SPT-CL J2332-5358
X-ray emission (in pink) along with Sunyaev-Zel'dovich effect (SZE) contours (in white) of the massive galaxy cluster SPT-CL J2332-5358, overlaid on an optical g-, r- and i-band colour composite image. The X-ray observations have been performed using XMM-Newton's new mosaic observing mode. The X-ray emission shown in the image corresponds to the 0.5-2.0 keV band. The SZE contours, from the South Pole Telescope galaxy cluster catalogue, are superimposed on an optical pseudo-colour image from the Blanco Cosmology Survey. The cluster has a mass of over 1015 solar masses, a temperature of about 9.3 keV (= 108 million Kelvin) and is located at redshift z=0.32. This composite image clearly show a large 'Brightest Cluster Galaxy' (BCG) within a few arc seconds from the peak of the X-ray emission; the SZE detection is also coincident with the observation in the other bands. The region depicted above is 6.4 x 6.4 arc minutes.
Credit: ESA/XMM-Newton; Background image: Blanco Cosmology Survey/NOAO/AURA/NSF; SZE contours: South Pole Telescope: NSF


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.

galaxy cluster SPT-CL J2342-5411
X-ray emission (in pink) along with Sunyaev-Zel'dovich effect (SZE) contours (in white) of the high-redshift galaxy cluster SPT-CL J2342-5411, overlaid on an optical g-, r- and i-band colour composite image. The X-ray observations have been performed using XMM-Newton's new mosaic observing mode. The X-ray emission shown in the image corresponds to the 0.5-2.0 keV band. The SZE contours, from the South Pole Telescope galaxy cluster catalogue, are overlaid on an optical pseudo-colour image from the Blanco Cosmology Survey. The cluster has a mass of about 3x1014 solar masses, a temperature of about 4.5 keV (= 52 million Kelvin) and is located at redshift z=1.08. This composite image clearly show a large 'Brightest Cluster Galaxy' (BCG) within a few arc seconds from the peak of the X-ray emission; the SZE detection is also coincident with the observation in the other bands. The region depicted above is 4.8 x 4.8 arc minutes.
Credit: ESA/XMM-Newton; Background image: Blanco Cosmology Survey/NOAO/AURA/NSF; SZE contours: South Pole Telescope: NSF
"Using XMM-Newton, we have independently detected two of the newly discovered clusters found by the SPT," says Robert Šuhada, who led the study. Using the X-ray data, the mass of both clusters could be estimated, leading to values of over 1015 solar masses and about 3x1014 solar masses, respectively. "One of the clusters is exceptionally massive, and it ranks among the hottest clusters ever observed," adds Šuhada.

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.

mosaic
This XMM-Newton image, obtained using the new mosaic mode of observations, shows the extent of the XMM-BCS survey. The survey targeted a patch of the sky of 14 square degrees, about 70 times the area of the full Moon - the angular size of the Moon is also shown for comparison. The green circles, with a radius of 4 arc minutes each, mark the positions of the two galaxy clusters SPT-CL J2332-5358 and SPT-CL J2342-5411. More than 100 other galaxy clusters have been found in this field. This false-colour image was constructed from X-ray surface brightness images in 3 bands: 0.3-0.5 keV (red), 0.5-2.0 keV (blue) and 2.0-4.5 keV (green), respectively. Most of the - over 3000 - point sources visible in the fields are Active Galactic Nuclei (AGN). Regions A, B and C mark the three large fields covered by mosaic mode observations, each amounting to about 2.7 square degrees, for a total of about 8 square degrees. Each of the three mosaics consists of 19 stable pointings, each with a 3.5 ks exposure, and the slews between them, for a total time of about 90 ks per mosaic. Region F covers about 6 square degrees and identifies the deeper core of the survey, consisting of 42 individual and partially overlapping pointings; each pointing corresponds to a 12 ks long exposure. As a result of the new mosaic observing mode, it has been possible to survey an area of the sky (regions A, B and C) larger than the original survey area (region F) in only a fraction of the time that was required to observe the latter using the standard mode.
Credit: ESA/XMM-Newton.
This result opens a new window to probe galaxy clusters to very high redshifts, which will be exploited by future missions examining different regions of the electromagnetic spectrum. One of the scientific goals of ESA's Planck mission, which is currently scanning the whole sky in microwaves, is to detect about 1000 galaxy clusters through their SZE signal imprinted on the CMB. The Euclid mission, a candidate Cosmic Vision M-class mission, is expected to detect a large number of clusters in optical and near-infrared wavelengths, thanks to its wide field of view, and to identify their redshifts. This first discovery is thus a preview of future galaxy cluster surveys and of the exciting scientific results they will bring, in the process expanding our knowledge about the evolution of cosmic structure.





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
external link Astronomy & Astrophysics Letters 514, L3 (2010)


Other press releases:
external linkESA press release


Contact:
internal link Dr. Hannelore Hämmerle
Press Officer
Max-Planck-Institut für extraterrestrische Physik
phone: +49 89 30000-3980
email: hanneh@mpe.mpg.de
internal link Dr. Hans Böhringer
Max-Planck-Institut für extraterrestrische Physik
phone: +49 89 30000-3347
email: hans.boehringer@mpe.mpg.de


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