HEA > ASCA > Scientific Results > Dark cluster

(6) Discovery of a dark cluster of galaxies

One of the important predictions of the theory of general relativity is that a gravitational force distorts a light path from a straight line. In fact, distant quasars often show multiple images and this is interpreted as due to the "gravitational lens" effect.

Gravitational lens happens when the line of sight from an observer to a quasar suffers from a gravitational bending effect from a large gravitational mass between them, several different light paths between them.

The MG2016+112 quasar is one such gravitationally lensed object discovered as a triple radio source. Two galaxies were found in the direction of the quasar and one had cosmological redshift, z, of about 1.

A model calculation suggested that the quasar might be being lensed by a cluster of galaxies centered in a distant galaxy. However, no evidence of the existence of the postulated cluster had been found in spite of deep optical and infrared searches.

The dark lensing object could be a bright X-ray source, since it can accumulate the intergalactic medium by its strong gravitational field, and heat the accumulated gas up to a temperature of a few tens millions of degrees due to the gravitational energy release.

In fact, X-ray observations have revealed that clusters of galaxies are generally luminous sources of X-rays, which are emitted from the very hot gas filling the intra-cluster space. The total mass of hot gas in a cluster galaxy is now known to exceed the mass of galaxies composing the cluster, on average.

Motivated by these observational results, a deep observation of MG2016+112 with ASCA was performed. A new X-ray source (referred to as AXJ2019+1127) was discovered in the direction of the lens system.

The X-ray spectrum of AXJ2019+1127 obtained by ASCA. An emission line feature is clearly seen at 3.5 keV which matches the redshifted Fe K line from a cluster of galaxies at z=1. The overall spectrum is consistent with X-ray emission from hot gas with a temperature of about 100 million degrees.

Following the ASCA observation, another deep X-ray observation of the quasar direction was performed by the German X-ray astronomy satellite ROSAT. The figure on the left shows the X-ray image obtained with ROSAT.

The X-ray source is clearly extended and the maximum surface brightness is consistent with the position of the optically discovered galaxy, which is thought to be in the center of the lensing cluster of galaxies.

The above results clearly imply that AXJ2019+1127 is a cluster of galaxies with z=1. This is the most distant cluster of galaxies discovered in X-rays so far. Since the X-ray emission provides the best evidence that clusters are gravitationally bound entities, this confirms the most distant cluster of galaxies.

Another spectacular result of this discovery is the high iron abundance detected in the intra-cluster medium of AXJ2019+1127. Although the margin of error concerning the abundance is quite large, it can safely be said that the iron content in this cluster is at least as high as that of nearby clusters of galaxies.

The detection of the large iron content at high redshift sets a new limit for the epoch of metal enrichment.

According to our current understanding, metal enrichment originates in the stars in galaxies composing the cluster. But it is very puzzling to detect such a high iron abundance in this "dark cluster", which is very poor in its galaxy content according to deep optical and infrared searches.

AXJ2019+1127 provides us with several difficult questions for current cosmological theories to answer.

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ISAS/JAXA Department of High Energy Astrophysics