ROSAT Discovers Unique, Distant Cluster of Galaxies
Brightest X-ray Cluster Acts as Strong Gravitational Lens
19 June 1995
Based on exciting new data obtained with the ROSAT X-ray satellite and a ground-based telescope at the ESO La Silla Observatory, a team of European astronomers  has just discovered a very distant cluster of galaxies with unique properties. It emits the strongest X-ray emission of any cluster ever observed by ROSAT and is accompanied by two extraordinarily luminous arcs that represent the gravitationally deflected images of even more distant objects. The combination of these unusual characteristics makes this cluster, now known as RXJ1347.5-1145, a most interesting object for further cosmological studies.
DISCOVERY AND FOLLOW-UP OBSERVATIONS
This strange cluster of galaxies was discovered during the All Sky Survey with the ROSAT X-ray satellite as a moderately intense X-ray source in the constellation of Virgo. It could not be identified with any already known object and additional ground-based observations were therefore soon after performed with the Max-Planck-Society/ESO 2.2-metre telescope at the La Silla observatory in Chile. These observations took place within a large--scale redshift survey of X-ray clusters of galaxies detected by the ROSAT All Sky Survey, a so-called ``ESO Key Programme'' led by astronomers from the Max-Planck-Institut fur Extraterrestrische Physik and the Osservatorio Astronomico di Brera. The main aim of this programme is to identify cluster X-ray sources, to determine the distance to the X-ray emitting clusters and to investigate their overall properties.
These observations permitted to measure the redshift of the RXJ1347.5-1145 cluster as z = 0.45, i.e. it moves away from us with a velocity (about 106,000 km/sec) equal to about one-third of the velocity of light. This is an effect of the general expansion of the universe and it allows to determine the distance as about 5,000 million light-years (assuming a Hubble constant of 75km/sec/Mpc). In other words, we see these galaxies as they were 5,000 million years ago.
Knowing the intensity of the X-ray emission as measured by ROSAT and also the distance, the astronomers were then able to estimate the total X-ray energy emitted by this cluster. It was found to be extremely high , in fact higher than that of any other cluster ever observed by ROSAT. It amounts to no less than 1.5 million million times the total energy emitted by the Sun.
It is believed that this strong X-ray emission originates in a hot gas located between the galaxies in the cluster. The high temperature indicates that the components of the gas move very rapidly; this is related to the strong gravitational field withinthe cluster.
THE GRAVITATIONAL ARCS
To their great surprise and delight, the astronomers also discovered two bright arcs, 5 - 6 arcseconds long and symmetrically placed about 35 arcseconds to the North-East and South-West of the brightest galaxies in the cluster (see the photo). They were detected on exposures of only 3 minutes duration with the 2.2-metre telescope and are among the brightest such arcs ever found.
At the indicated distance, the arcs are situated at a projected distance of about 500,000 light-years from the centre of the cluster. It is an interesting possibility that the two arcs may in fact be two images of the same, very distant galaxy, that is situated far beyond RXJ1347.5-1145 and whose light has been bent
and split by this cluster's strong gravitational field.
This strange phenomenon was first discovered in the late 1970's and is referred to as gravitational lensing. Quite a few examples are now known, in most cases in the form of double or multiple images of quasars. About three dozen cases involve well visible galaxy clusters and elongated arcs, but few, if any, of these arcs are as bright as those seen in the present cluster.
This particular arc configuration enables a very accurate determination of the total mass of the cluster, once the distance of the background galaxy has been measured (by obtaining spectra of the arcs and measuring their redshift). The masses of galaxy clusters are important for the determination, for instance of the mean density and distribution of matter in the universe. This is because these clusters are the most massive, clearly defined objects known and as such trace these parameters in the universe on very large scales.
Another possibility to derive the cluster mass is offered by X-ray observations, because the distribution of the hot, X-ray emitting gas traces the gravitational field of the cluster. Recently, in some clusters there has been a discrepancy between the mass determined in this way and that found from gravitational lensing
effects. The team of astronomers now hopes that follow-up X-ray observations of RXJ1347.5-1145 will help to solve this puzzle.
Moreover, the combination of extremely high X-ray brightness and the possibility to perform a rather accurate mass determination by the gravitational lensing effect makes this particular cluster a truly unique object. In view of the exceptional X-ray brightness, a very high mass is expected. The exact determination will be possible, as soon as spectra have been obtained of the two arcs. Contrary to what is the case in other clusters, this will not be so difficult, due to their unusual brightness and their ideal geometrical configuration.
 This is a joint Press Release of ESO and the Max-Planck-Society. It is accompanied by a B/W photo.
 The investigation described in this Press Release is the subject of a Letter to the Editor which will soon
appear in the European journal Astronomy & Astrophysics, with the following authors: Sabine Schindler (Max-Planck-Institut fuer Extraterrestrische Physik and Max-Planck-Institut fuer Astrophysik,
Garching, Germany), Hans Boehringer, Doris M. Neumann and Ulrich G. Briel (Max-Planck-Institut fuer Extraterrestrische Physik, Garching, Germany), Luigi Guzzo (Osservatorio Astronomico di Brera, Merate, Italy), Guido Chincarini (Osservatorio Astronomico di Brera, Merate, and Dipartimento di Fisica, Universita di Milano, Italy), Harald Ebeling (Institute of Astronomy, Cambridge, U.K.), Chris A. Collins (School of Chemical and Physical Sciences, John-Moores University, Liverpool, U.K.), Sabrina De Grandi (Dipartimento di Fisica, Universita di Milano, Italy), Peter Shaver (ESO, Garching, Germany) and Giampaolo Vettolani (Istituto di Radioastronomia del CNR, Bologna, Italy).
 The total X-ray energy emitted by RXJ1347.5-1145 is (6.2 +-0.6) 10^45 erg s-1 in the range 0.1--2.4 keV.
BRIGHT X-RAY CLUSTER ACTS AS STRONG GRAVITATIONAL LENS
This is an optical image of the most luminous ROSAT cluster of galaxies RXJ1347.5-1145, obtained with the MPG/ESO 2.2-metre telescope at the La Silla Observatory. The image shows the central part of the
cluster with two bright gravitational arcs.
The arcs are 5 - 6 arcseconds long and located about 35 arcseconds to the North-East and South-West of the brightest galaxies in the cluster. They were detected on exposures of only 3 minutes duration with the 2.2-metre telescope and are among the brightest such arcs ever found.
At the distance of the cluster, approx. 5,000 million light-years, the arcs are situated at a projected distance of about 500,000 light-years from the centre of the cluster. It is an interesting possibility that the two arcs may in fact be two images of the same, very distant galaxy, that is situated far beyond RXJ1347.5-1145 and whose light has been bent and split by this cluster's strong gravitational field.
This is photo eso9508a which accompanies eso9508. It may be reproduced, if credit is given to the European Southern Observatory and the Max-Planck-Society.