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.
 

Fig.1: The first two arcs which were discovered in 1986. Left is the arc in the cluster A370 (z=0.37), whose redshift was measured to be 0.735; right is the arc in Cluster C12244 (z=0.31). The spectroscopy of this arc yielded a redshift of 2.24, which at that time represented the most distant galaxy. Bright knots on the arcs clearly show the structure of the intrinsic distribution of brightness of the galaxies, whose images are strongly distorted. The influence of individual cluster galaxies on the detailed morphology of the arc in A370, which coils around these galaxies, can also be recognized.

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.
 

Fig.2: A deep multi-color image of the galaxy cluster A2218 (z=0.175) taken with the HST. The center of the cluster is near the bright galaxy to the left and below the middle of the picture. It is surrounded by several arcs. One of these arcs (A0) has a redshift of 2.515; another image of the galaxy to which the arc belongs can be seen near A2. A second concentration of mass is to the right above the middle of the picture. Here, too, several arcs appear around it. It should be noted that some of these arcs are very thin. Thus in order to study them in detail, the HST is indispensible. In particular, multiple images can be identified by a detailed comparison of the morphology of the light distribution as well as by the spectral properties of their light (i.e. of the colors). A cluster containing so many lens systems allows us to construct a very detailed model of the mass.

Fig.3: The cluster A2390 (a picture taken by HST is shown here; however, in this case the colors show only the light intensity, not the spectral distribution) seems to possess less spectacular arcs than A2218. However, next to the thick arc with z=0.913 one can distinctly see other arcs. It is notable that two multiple-image systems (indicated by the letters A and B, respectively) can be seen in this image. From a model of the mass, a high redshift is predicted for both systems. Indeed, in the meantime the redshifts have been measured, yielding 4.04 for System A and 4.05 for System B.

Fig.4: The cluster of galaxies Cl0024+16 at z=0.39 contains an impressive system of arcs, which are seen as relatively blue sources, compared to the reddish galaxies belonging to the cluster itself. The three parts of the arc seen to the left of the center of the galaxy cluster gave this system the name `triple-arc'. The middle part of this arc is relatively short, which is due to the perturbing influence of closely positioned cluster galaxies. To the right of the cluster center one can see an additional image of the corresponding source which gives rise to the triple-arc. In the lower left of the figure, this source (with redshift 1.63) has been reconstructed from models of the individual pieces of the arc. The close similarity of the reconstructed source morphologies is a clear proof of the fact that the arcs are indeed multiple images of the same background galaxy which has a fairly complicated intrinsic shape.

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.
 

Fig. 5: In this galactic cluster MS1512 (z=0.3), whose central galaxy can be seen in the middle of the picture, extensive spectroscopic examinations were undertaken. It was found that galaxy cB58 has a redshift of z=2.72, meaning it is located far behind the cluster. With this HST image, it first could be demonstrated that cB58 is an extended arc (this is difficult to recognize in the figure due to the limited dynamic range of the reproduction). A detailed analysis of this picture revealed that there is an additional image of the galaxy belonging to cB58; it is indicated here with A2. Furthermore, using the morphology and color it was determined that B1-3, C1-3, W1-3 and WC each are multiple images of background galaxies. With them it was possible to make a very detailed model of the distribution of the mass of the cluster; in particular, from this model one deduces that cB58 is about 20 times brighter than the unlensed galaxy would be.

Fig. 6: A spectrum of cB58 taken with the Keck telescope. The fine details in the spectrum can be seen here with a surprisingly low level of noise. This was made possible because in the observation, in addition to the largest optical telescope, the galaxy cluster with its light-magnification property was used. In order to obtain a spectrum of a similar quality from the unlensed source, one would have to observe 400 times longer!

Fig. 7: On an HST image of the galaxy cluster Cl1358+62, the lensed image of a background galaxy with z=4.92 was detected; it is seen here as a reddish arc below and to the right of the cluster center. The upper right panel shows an enlarged version of this lensed galaxy, and the lower right panel shows a reconstruction of the unlesned source, obtained from a lens model of the cluster.

Gravitational Lenses I   : Galaxies as Lenses

Gravitational Lenses III: The Weak Lensing Effect