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> 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
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
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
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
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.
Previous: (5) Explanation
of chemical evolution in the universe
Next: (7) Distribution and
total mass of dark matter
ISAS/JAXA Department of High Energy Astrophysics
Last Modified: Wednesday, 21-Nov-2001 10:46:59 JST