The serendipitous cluster seen by XMM-Newton's EPIC cameras. Photo: ESA

Serendipity, or chance discovery, plays a big role in astronomy. Observing one celestial target astronomers often find, in a corner of their telescope's field of view, another interesting and perhaps unknown object. XMM-Newton has made one such discovery.

During the calibration of its science instruments, ESA's new X-ray space observatory was recently scanning a portion of our Milky Way galaxy. In one of the images taken by the EPIC X-ray cameras, a new object was discovered, an impressive X-ray source of unexpected brightness (to the right in picture above). It contrasts with the other point sources in the field of view.

Previous pictures of this region taken by the ROSAT observatory had shown nothing similar. This can be confirmed by viewing the same XMM image data solely in ROSAT's lower energy range (picture below). Even in this range, low energy X-rays from this object have been absorbed by all the gas and dust in the Milky Way. The higher energy rays accessible to XMM are much less sensitive to absorption. This, together with the greater power of the XMM telescopes made the serendipitous discovery possible.

XMM-Newton's great appetite for X-rays and particularly those at higher energies reveal the newcomer in all its splendour. When its X-ray spectrum, showing the energies of the photons, was examined an unmistakable feature was evident: the presence of iron in a hot gas.

Same sky region seen in the ROSAT low-energy range. Photo: ESA

Furthermore, it was clear that this prominent iron line was shifted to lower than normal values (the dashed vertical line). This indicates a "red shift" caused by the expansion of the Universe, a Doppler effect much as the lowering pitch of a police car siren means it is moving away.

Measurement of this red shift shows that the object lies at a distance of more than 1000 million light years beyond the Milky Way. This red shift, together with the extended nature of the source, tells astronomers that it is probably a distant cluster of galaxies.

Clusters of galaxies, the biggest things in the Universe consisting of hundreds of galaxies bound together by their mutual gravitational attraction, are of great interest for cosmologists. They are indications on how the early Universe evolved, when matter initially smoothly distributed started to clump together. The study of clusters can also indicate how the Universe is actually evolving.

Optical pictures of a cluster of galaxies show the galaxies themselves and not the hot gas that often lies between them. In contrast, an X-ray picture mainly shows these hot gases, which at millions of degrees shine brightly in X-rays, and some of the constituent galaxies more faintly.

In the case of XMM-Newton's observation revealing the object, the dense strip of stars in our Galaxy makes it extremely difficult to detect the cluster with normal optical telescopes. This chance discovery confirms that XMM-Newton will be extremely efficient to map out the distribution of clusters beyond the Milky Way.