Galaxies in the Universe are often to be found in clusters. Our own Milky Way is a member of a small cluster of galaxies, the Local Group. But clusters also exist that contain thousands of galaxies. XMM-Newton has obtained a remarkably vivid mosaic of one of the most famous of these, the Coma cluster.

Coma cluster of galaxies seen in the 0.3-2.0keV energy band using XMM-Newton's EPIC-pn camera. Photo: ESA

This cluster is situated in the northern sky constellation Berenices. The constellation also includes the Virgo cluster, 50 million light years away from us. Coma is considerably more distant, some 280 million light years away.

In a region of the sky covering more than 4 degrees, and with a true diameter of at least 20 million light years, the Coma cluster is an aggregation of more than 3000 galaxies. Most of its brightest members are elliptical galaxies or lenticular galaxies.

The Coma cluster - described in 1785 by William Herschel as "the nebulous stratum of Coma Berenices...almost everywhere equally rich in fine nebulae" - has been extensively studied to understand the way in which galaxies interact and how such galaxy clusters are formed.

In the 1960s such clusters were detected as X-ray sources. The X-ray emission arises from hot (10-100 million degree) gas within the cluster. This "intracluster medium" is probably primordial, either part of the cluster when it formed or accreted afterwards. It also contains processed gas indicated by iron line emission.

NGC 4839 using the EPIC-MOS1 and EPIC-MOS2 camera. Photo: ESA

Clusters are also believed to contain, in addition to the baryonic ("ordinary") matter, another component, dark matter whose properties are not well understood and whose presence can probably only be detected by its gravitational influence.

XMM-Newton observed the Coma cluster between 29 May and 22 June during the performance verification phase of its science instruments. The longest effective observation duration was practically 10 hours, with an average of nearly 5 hours. The aim was to verify the observatory's ability to map large extended X-ray sources. The data were analysed by a large team of investigators led by Monique Arnaud of the Service d'Astrophysique, CEA Saclay France. The mosaic is the result of work by Ulrich Briel, of the Max Planck Institute for Extraterrestial Physics, Garching Germany.

The mosaic is composed of 12 partially overlapping pointings of the EPIC-pn camera and it maps the full extent of the Coma cluster. With the telescope's 5 arcsec resolution, the imaging cameras produce pictures in different energy bands which are extremely rich in details. In the shown image, produced with the pn-camera in the band from 0.3 to 2.0 keV, a huge number of galaxies belonging to the cluster are detected and some tentative identifications are provided.

The mosaic allows the identification of many galaxies in the X-ray domain of the spectrum. For example, the X-ray emission is clearly visible from the two very bright giant elliptical galaxies, NGC 4889 and particularly NGC 4874, lying either side of the heart of the cluster, and many individual sources are seen in Coma's outlying regions

Coma cluster observed by ROSAT in 1993. Photo: ESA

The XMM image has a vivid clarity compared to previous views, taken for instance by the ROSAT X-ray observatory some 7 years ago.

Astrophysicists have been particularly interested in one of the Coma cluster's substructures, the fainter group of galaxies (seen to the lower right of the mosaic) - including NGC 4839 - which appears to be merging into the brighter central region.

Detailed examination of NGC 4839 using the EPIC MOS cameras shows its tail-like structure pointing away from the Coma cluster. The orientation of NGC 4839 in the optical coincides with the orientation of the tail.

Subclusters are of particular interest because they are the building blocks from which clusters of galaxies are being assembled.