Gravitational Lenses II:
Galaxy Clusters as Lenses
What are Galaxy Clusters?
Galaxies in the universe are not randomly distributed. It has long been known that galaxies tend to concentrate in groups. The distribution of the bright (and thus nearby) galaxies in the sky shows a strongly correlated structure with areas of overly-dense concentrations of galaxies. The densest areas are galaxy clusters. Often a hundred or more galaxies are found within a sphere whose radius is barely larger than the distance of our Milky Way to the Andromeda galaxy. These areas are called galaxy clusters, within which the galaxies are bound gravitationally. By studying the motion of galaxies in clusters, Fritz Zwicky already found in 1933 that such clusters must contain much more mass than can be seen in the galaxies. This was the first indication of the existence of Dark Matter. Furthermore, galaxy clusters contain a diffusely-distributed, very hot gas (more than 10 million degrees Celsius) which is visible in the x-ray region of their spectrum. These high temperatures clearly tell us that the gas is trapped in a very deep potential well, again allowing us to conclude that Dark Matter is involved.
Galaxy clusters are the largest bound objects in the universe
and, cosmologically speaking, they are very young. The timescale a galaxy
needs to transverse the cluster is not much shorter than the age of the
universe (in comparison: our sun has already orbited the center of the
Milky Way about 100 times). Thus, galaxy clusters are particularly interesting
objects since they have not yet "forgotten" the conditions under which
they were formed. To some extent, one can see how they were created. Therefore
galaxy clusters form a bridge between cosmology and astrophysics.
Glowing Arcs in the Sky
In the mid-1980s, the existence of long, arc-like structures was discovered in some galaxy clusters. These banana-shaped forms are close to the centers of clusters and are often strongly curved. The term "arcs" reflects their morphology quite well. At first, the nature of the arcs was controversial until their distance could be determined by spectroscopy. They are considerably farther away from us than are the clusters to which they belong, meaning they are not associated with them physically. Rather, they are images of background galaxies which are extremely distorted by the gravitational lens effect.
In the meantime, several examples of such arcs have been
found. Whenever a deep (long exposure) image of a very massive galactic
cluster is taken with a sufficiently high resolution, one can detect the
presence of arcs, though not always so impressively as the examples shown
here. From the shape and the location of the arcs relative to the center
of the cluster, one can directly determine the mass in the inner region
of the cluster. In order for such an extreme distortion to occur, the arc
must be located very close to the Einstein radius of the cluster. Results
achieved with other methods are confirmed: Clusters of galaxies are, to
a large extent, composed of Dark Matter. The mass of the galaxies
and of the hot gas make up only about 15% of the total mass of the cluster.
Hubble takes a look
After the Hubble Space Telescope (HST) was repaired in 1993 and high-resolution pictures could be taken, the study of arcs intensified. Despite their lengths, many of the arcs are so thin that they could not be resolved even with the HST. The length-to-width ratio of some arcs exceeds 30! This extreme distortion of images of background galaxies is accompanied by a respective magnification in the observed brightness: Arcs are considerably brighter than the unlensed galaxies to which they belong would appear.
In addition to these "Giant Arcs" there are other lens
phenomena in clusters. As is easily imaginable, if there are some galaxies
whose images are greatly distorted, there are many more with a somewhat
lesser distortion. These small arcs were named arclets; their axis ratio
is typically 3 - 10. They can be found somewhat further away from the center
of the cluster than the arcs themselves. Furthermore, multiple images of
some background galaxies will be seen due to the lensing effect of the
galaxycluster. Using all this information, detailed models of the mass
distribution in the center of the galacxy cluster can be constructed. For
example, it was found that material in clusters is much concentrated than
has been deduced from x-ray observations. The distribution of Dark Matter
in the clusters is closely related to the population of light in the central
cluster galaxy. This allows one to draw conclusions with regard to the
history of the evolution of this galaxy.
Galaxies as Natural Telescopes
The large magnification in apparent brightness due to
the lens effect can be used to investigate sources in much greater detail
than would be possible without magnification. Since very distant galaxies
represent very faint sources, the magnification is a welcome source of
assistance. The arc in cluster Cl2244 (see Fig. 1) was an image of the
first normal galaxy found to have a redshift of >2. In the meantime, the
spectroscopy of arcs and arclets yielded many galaxies with a higher redshift.
A large number of galaxies with redshifts between 2.5 and 4.5 have been
found, but even with the largest telescopes available, an extremely long
exposure time is needed to obtain a detailed spectrum of such galaxies.
A galaxy which will is magniifed by a factor of 20 needs only 1/400th the
time for a spectrum of a similar quality as does the corresponding unlensed
galaxy! With this natural bonus, it is possible to study the chemical composition
of galaxies which are only around 20% of the age of the universe.
Gravitational Lenses I : Galaxies as Lenses
Gravitational Lenses III: The
Weak Lensing Effect