ESA's orbiting X-ray observatory XMM-Newton has discovered the most massive cluster of galaxies seen in the distant Universe until now. The galaxy cluster is so big that there can only be a handful of them at that distance, making this a rare catch indeed. The discovery confirms the existence of dark energy.

The newly-discovered monster, known only by the catalogue number 2XMM J083026+524133, is estimated to contain as much mass as a thousand large galaxies. Much of it is in the form of 100-million-degree hot gas. It was first observed by chance as XMM-Newton was studying another celestial object and 2XMM J083026+524133 was placed in a catalogue for a future follow-up.

Georg Lamer, Astrophysikalisches Institut Potsdam, Germany, and a team of astronomers discovered the record-breaking cluster as they were performing a systematic analysis of the catalogue. Based on 3500 observations performed with XMM-Newton's European Photon Imaging Camera (EPIC) covering about 1% of the entire sky, the catalogue contains more than 190 000 individual X-ray sources.

The team were looking for extended patches of X-rays that could either be nearby galaxies or distant clusters of galaxies.

J083026+524133 stood out because it was so bright. While checking visual images from the Sloan Digital Sky Survey, the team could not find any obvious nearby galaxy in that location. So they turned to the Large Binocular Telescope in Arizona and took a deep exposure.

Sure enough, they found a cluster of galaxies. So the team calculated a distance of 7.7 thousand million light-years and the cluster's mass using the XMM-Newton data. This was not a surprise because XMM-Newton is sensitive enough to routinely find galaxy clusters at this distance. The surprise was that the cluster contains a thousand times the mass of our own galaxy, the Milky Way.

"Such massive galaxy clusters are thought to be rare objects in the distant Universe. They can be used to test cosmological theories," says Lamer. Indeed, the very presence of this cluster confirms the existence of a mysterious component of the Universe called dark energy.

No one knows what dark energy is, but it is causing the expansion of the Universe to accelerate. This hampers the growth of massive galaxy clusters in more recent times, indicating that they must have formed earlier in the Universe. "The existence of the cluster can only be explained with dark energy," says Lamer.

Yet he does not expect to find more of them in the XMM-Newton catalogue. "According to the current cosmological theories, we should only expect to find this one cluster in the 1% of sky that we have searched," says Lamer.

In other words, the team have found a cosmic 'needle in a haystack'.

related report
Discovery Of The Most Massive Cluster Of Galaxies
XMM-Newton has discovered a rare, very massive cluster of galaxies at a distance of about 7700 million light years (or z about 1).

The object, designated 2XMM J083026+524133, was discovered during a systematic analysis of the 2XMM X-ray source catalogue. In a paper, to appear in Astronomy and Astrophysics, Georg Lamer and colleagues present the discovery and analysis of this exceptional cluster of galaxies.

The 2XMM catalogue is based on about 3500 observations performed with XMM-Newton's EPIC cameras between February 2000 and March 2007. The catalogue covers about 360 square degrees (about 1% of the entire sky) and contains about 192 000 individual X-ray sources.

A team of astronomers at the Astrophysikalisches Institut Potsdam, led by Georg Lamer, searched the catalogue for distant and bright clusters of galaxies. These large collections of gravitationally bound galaxies have a tenuous but very hot intra-cluster gas component with a temperature of up to 100 million Kelvin, causing thermal emission at X-ray wavelengths.

Lamer et al. selected all extended sources in the 2XMM catalogue and correlated these with data from the Sloan Digital Sky Survey (SDSS) to search for optical counterparts. About a third of the sky covered in the 2XMM catalogue is also covered by the SDSS. Within this region the majority of the selected X-ray sources were found to have an optical counterpart in the SDSS data.

However, for a small number of X-ray sources no visible optical counterpart was detected. These sources were considered candidates for distant clusters of galaxies at redshifts above about 0.8. The large distance would render the optical light too faint to have been detected by the SDSS.

The brightest of the selected X-ray sources without an SDSS counterpart was 2XMM J083026+524133. The source was observed for an accumulated total of nearly 24 hours by XMM-Newton, gathering sufficient X-ray photon counts for a good X-ray spectrum to be obtained.

Lamer et al. determined the best fit spectrum using a plasma model for a cluster of galaxies. The fitted spectrum implies a redshift of 0.99 +/- 0.03 and a temperature of 8.2 +/- 0.9 keV or about 95 million K for the intra-cluster gas.

This makes 2XMM J083026+524133 the hottest, most X-ray bright cluster of galaxies at redshifts z = 1, with a bolometric luminosity Lbol of 1.8 � 1045 erg s-1. Using a mass model and the measured temperature, Lamer et al. also derive a mass of about 5.6 � 1014 Msun for this cluster of galaxies, equal to about 1000 times the mass of our Milky Way galaxy.

Ground-based follow-up
Follow-up observations were performed in May 2008 with the world's largest telescope, the Large Binocular Telescope (LBT) on Mount Graham in southeast Arizona, USA. Two broad filters were used to make deep observations of the field around 2XMM J083026+524133 in the R band (Sloan r' about 550-690 nm) and Z band (Sloan z about 850-1000 nm (upper cut-off defined by the detector and not by the filter)).

These optical and near-infrared images provided the sought after counterpart and support the identification of 2XMM J083026.2+524133 as a cluster of galaxies (Figure 1) because:

+ there is an increased density of red galaxies within the X-ray contours of the extended source from the 2XMM catalogue

+ the magnitudes and colours of these galaxies are consistent with a redshift of about 1 Cosmological models

Clusters of galaxies at a redshift of about 1 or higher are routinely identified with XMM-Newton data and in dedicated surveys with other observatories. However, 2XMM J083026.2+524133 is a factor of about 100 brighter in X-rays than the majority of the other known clusters of galaxies at these redshifts, making it an important find.

The evolution of the number of high-mass, high-luminosity clusters of galaxies, similar to 2XMM J083026.2+524133, over the age of the Universe, strongly depends on cosmological parameters. Large X-ray surveys such as the 2XMM catalogue can provide the necessary observational constraints on these parameters through the observed number of these massive clusters at large distances.

The counts can be compared with the number of massive clusters of galaxies in the local Universe to characterise their number evolution with time. This in turn has implications for the validity of cosmological models.