XMM discovers monster galaxy cluster
BY DR EMILY BALDWIN
ASTRONOMY NOW

Posted: August 27, 2008

ESA's orbiting X-ray observatory XMM-Newton has discovered a cosmic needle in the haystack of space: the most massive galaxy cluster ever seen in the distant Universe, and one which confirms the existence of dark energy.

The image in which 2XMM J083026+524133 was dicovered, taken by the EPIC camera onboard ESA’s XMM-Newton observatory. 2XMM J083026+524133 was captured unexpectedly to the right of the image while observing the bright spot in the upper left, an active galaxy. Image: ESA XMM-Newton/EPIC (G. Lamer).

The XMM science team discovered the record-breaking cluster as they were performing a systematic analysis of the satellite’s catalogue – which covers just one percent of the sky but over 190,000 individual X-ray sources – in the search for galaxy clusters at redshifts (z) of 1, or a distance of around 6 billion light years.

A monstrous cluster, assigned the tag of 2XMM J083026+524133, shone out like a beacon among the cluster candidates, although a check with visual images from the Sloan Digital Sky Survey did not confirm any obvious nearby galaxy in that location. So the astronomers turned to the Large Binocular Telescope (LBT) in Arizona and took a deep exposure, which uncovered a cluster of galaxies located at a distance of 7.7 billion light-years. XMM is sensitive enough to routinely find galaxy clusters at this distance, but the icing on the cake was that the cluster was found to contain a staggering thousand times the mass of our own Milky Way Galaxy, which weighs in at around 5.8x10^11 solar masses. The cluster is made up of 100 million degree hot gas and its mass was estimated by using the known correlation of X-ray temperature and mass of individual galaxies.

“The most massive cluster known in the Universe is RX J1347.5-1145 at z=0.451 with an estimated mass of 1.4x10^15 solar masses,” Georg Lamer of the Astrophysikalisches Institut Potsdam in Germany tells Astronomy Now. “2XMM0830 is the most massive cluster at z=1 and beyond."

The optical image that confirmed that 2XMM J083026+524133 is a distant cluster of galaxies, taken by the Large Binocular Telescope in Arizona. The X-ray emission from the cluster of galaxies is shown in blue at the centre of the image and individual galaxies in the cluster are the small dots inside the blue glow. Image: ESA XMM-Newton/EPIC, LBT/LBC, AIP (J. Kohnert).

Such massive galaxy clusters, like 2XMMJ0830, are thought to be rare objects in the distant Universe so they are invaluable in testing cosmological theories, and the very presence of this cluster confirms the existence of a mysterious component of the Universe called dark energy. No one really knows what dark energy is, but it is causing the expansion of the Universe to accelerate while hampering the growth of massive galaxy clusters in more recent times, implying that galactic monsters formed in a more youthful Universe.

"The existence of the cluster can only be explained with dark energy," says Lamer. “There is no other good explanation for the existence of massive clusters at z=1. Clusters like 2XMM0830 must have been 200 times rarer per volume at z=1 than they are now. But it has to said that the term ‘dark energy’ simply explains some yet unknown physics behind the observation that the expansion of the Universe accelerates.”

According to current cosmological theories, astronomers should only expect to find this one cluster in the one percent of sky that they searched, making the observation an extremely lucky catch.

“The project is not finished yet, we hope to obtain more imaging of XMM clusters with the LBT soon, but we would be very lucky if we found another cluster as massive as 2XMM 0830," adds Lamer. "Maybe an all-sky survey could find a cluster with 10^15 solar masses." A project aimed at the discovery of such clusters is the eROSITA all sky X-ray survey to be flown on the Russian satellite Spectrum-X-Gamma, due for launch in 2011.

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