Planck spies a new supercluster
As the light from the Cosmic Microwave Background (CMB) travels through the Universe, it encounters various objects on its 13.7 billion year journey to Planck's detectors. But one particular type of object has a very special effect. Massive galaxy clusters contain very hot ionised gas, which means there are lots of extremely energetic and fast-moving electrons flying around. Becaus of gravity, these are gathered close to the centre of the cluster. When the CMB light encounters these electrons, it actually gains a little bit of energy. This increase in energy of the light changes the way it looks over a range of frequncies. Specifically, there is a little less light at lower frequencies and more at higher frequencies, when compared with what is expected from the CMB. This effect is known as the "Sunyaev-Zel'dovich" effect.
This effect is shown in the images below of the galaxy cluster Abell 2319, 800 million light years away in the constellation of Cygnus the Swan. The blue colours at low frequencies show there is slight less light, while the red colours at high frequency show slightly more light. At an intermediate frequency, 217 GHz, there is actually no effect and the original CMB can be seen through the cluster.
Images of the galaxy cluster Abell 2319 at seven of Planck's frequencies. Click here, or on the image, for more information. Image credit: ESA / HFI & LFI Consortia
The Coma Cluster as seen by Planck (top left) and ROSAT (top right). The bottom two panels show the same images superimposed on visible light images of the same cluster. Click here, or on the image, for more information. Image credits: ESA / LFI and HFI Consortia (Planck image); MPI (ROSAT image); NASA/ESA/DSS2 (visible image). Acknowledgement: Davide De Martin (ESA/Hubble).
The gas and electrons in the centre of galaxy clusters are so hot that they are emitting X-rays. The image on the right shows the Coma Cluster, one of the most densely populated galaxy clusters known. The top left panel shows the Sunyaev-Zel'dovich effect seen by Planck, while the top right panel shows X-ray emission from the gas - the similarity between the two images is a very strong indication that they are due to the same object. The bottom two panels show the same images overlaid on visible light images of the cluster. The x-ray images also look at slightly different properties of the gas, and so by combining the two sets of data much more can be discovered about these clusters.
The Coma Cluster was known about long before it was seen in x-rays or microwaves. But the Sunyaev-Zel'dovich effect allows Planck to see clusters that have not been seen before. One property of the effect is that it doesn't matter how far away the galaxy cluster is. While the images at x-ray and visible light are much fainter for more distant objects, the Sunyaev-Zel'dovich effect doesn't care, allowing us to see clusters of galaxies much further away. Combined with the fact that Planck is scanning the sky at nine different frequencies, this makes the satellite extremely powerful tool for
The newly-discovered supercluster as seen by Planck via the Sunyaev-Zel'dovich effect (left) and XMM-Newton in x-rays (right). The XMM_Newton image clearly shows that there are at least three individual clusters. In both images, the contours show the x-ray brightness. Click here for a version without annotations. Image credits: ESA / HFI & LFI Consortia (Planck); ESA (XMM).
Images from Planck are compared with all-sky images from the ROSAT x-ray satellite, to build up a catalogue of possible clusters. To check that they are clusters, they can then be observed with XMM-Newton, an ESA x-ray satellite currently in orbit. An images of one of the first such discoveries has now been released, shown on the right. The Planck image shows the Sunyaev-Zel'dovich effect, while the XMM-Newton shows the X-rays. The object is too small to be seen properly by Planck, and is little more than a fuzzy blob, but the higher resolution x-ray images shows that this isn't just a single cluster. This object is in fact a "supercluster", made up of at least three clusters - each of which contains hundreds or thousands of galaxies like our own Milky Way.
Many more such examples will be published in January 2011, in Planck's "Early Release Compact Source Catalogue".
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