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Most distant
galaxy cluster found

Posted: 10 May 2010

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A team of astronomers have identified the most distant galaxy cluster yet using near-infrared spectroscopy, placing it at a distance of 9.6 billion light years.

Galaxy clusters are the largest gravitationally bound objects in the Universe, containing tens to thousands of galaxies. Clusters are thought to have begun assembling between ten billion years ago and now, so finding coherent galaxy groupings at such early epochs reveals important information about the formation and evolution of structure during the Universe's first few billion years.

This image of cluster SXDF-XCLJ0218-0510 measures 3.4 arcminutes on each side; the arrows indicate galaxies that are likely located at approximately the same distance. The cluster emits X-rays as shown by the contours, and the circles show galaxies whose distances are accurately measured from the near-infrared observations and have been confirmed to be at 9.6 billion light years away.

Previously, galaxy cluster XMMXCS J2215.9-1738, discovered by ESA's XMM-Newton space telescope in 2006 at 9.2 billion light years, held the distance record. Now, a team of astronomers from Japan and Germany – Masayuki Tanaka of the Institute for the Physics and Mathematics of the Universe (IPMU), Alexis Finoguenov of the Max Planck Institute for Extraterrestrial Physics, and Yoshihiro Ueda of Kyoto University have smashed this record using the Subaru telescope's MOIRCS (Multi-Object InfraRed Camera and Spectrograph)'s near-infrared capabilities to find a cluster at 9.6 billion light years.

“Though we confirmed only several massive galaxies at that distance, there is convincing evidence that the cluster is a real, gravitationally bound cluster,” says Tanaka.

Follow-up observations were made with the orbiting X-ray telescope XMM-Newton to detect hot gas in the cluster and confirm its 9.6 billion light year distance. Though the number of the confirmed members is small, the combination of the X-ray detection and infrared observations confirms that the cluster hosts predominantly old, massive galaxies, thus demonstrating that the galaxies formed when the Universe was still very young.

"The main result of our study is the exact measurement of the distance to a cluster using the absorption and emission features in the galaxy spectra," Finoguenov tells Astronomy Now. "This is a very expensive measurement, but is the one required to confidently derive the Doppler shifts and the corresponding distance to an object."

Last year came the report of another cluster – JKCS 041, read our news story here) – located at 10.2 billion light years distant, but Finoguenov comments that this distance may be unreliable because it is based on galaxy colours, known as a photometric redshift, while the new cluster has a distance determined by measurements of the shifts of well-defined spectral features.

JKCS 041 is 190 million light years across and may also be one of the Universe's most distant clusters. In this Chandra X-ray image, X-ray emission is shown in blue. Image: NASA/CXC/INAF/S.Andreon et al.

"Photometric redshift is a widely used and well established technique and requires less detailed data than spectroscopic redshifts, so it is very useful for faint, distant galaxies," explains Ben Maughan of the University of Bristol, part of the team reporting the results on JKCS 041 last year. "However, it is certainly a less reliable measurement than a spectroscopic redshift, which require significantly more detailed observations that are challenging for very distant galaxies."

Maughan tells Astronomy Now that the lead author of the research on JKCS 041, Stefano Andreon, has been performing follow up observations to attempt to confirm the photometric redshift spectroscopically. "In the interim between our work and this new release it has also been reported by another group that there is some evidence for multiple structures along the line of sight to JKCS 041, which could complicate the distance measurement for JKCS 041, and possibly mean our photometric measurement is incorrect."

Meanwhile, the German-Japanese team members are continuing their search for yet more distant clusters. "If we have a larger number of clusters at such a distance, we can say something about the mass/energy content of our Universe," says Tanaka. "We can also say how uniform the early Universe was. It was very close to uniform, but not totally – small deviations from the uniformity result in the rich cosmic large-scale structure today."

Regardless of which cluster is located the furthest, it is clear that both will provide new insight into the processes that dominated cluster formation in the early Universe.

"I'm very excited by the work being done to push deeper into the Universe in our studies of galaxy clusters, and the more objects like these we find, the better we'll be able to understand how they form and understand the information they hold about the evolution of the Universe," adds Maughan.