Galaxy Clusters
when the Universe was Middle Aged

Scientists at the Max-Planck-Institute for Astrophysics are involved in a project to study clusters of galaxies when the Universe was only half of its present age. Along with a large international collaboration, the Max-Planck astronomers have measured the mass of galaxy clusters in the distant Universe and have used the Hubble Space Telescope to examine the shapes of the galaxies in exquisite detail.

Fig. 1: These four images show color images of the field around four clusters at different redshifts and with different masses. The brightest galaxy in each cluster is identified by the yellow arrow, and the cluster itself can be seen as an abundance of red galaxies around the brightest member. The white contours indicate lines of constant mass surface density in each cluster, as determined by a gravitational lensing analysis. In these plots, many concentric lines indicate a high mass. Although both clusters are comprised of many galaxies, only two of them have strong signals, demonstrating the large range of properties in our sample.

Galaxy clusters are the most massive quasi-equilibrium objects in the Universe and are the meeting places of the cosmos. Their deep potential wells are dominated by unseen dark matter, but contain a cosmologically representative baryon fraction in the form of galaxies and intergalactic gas. These are trapped in a virialized state, with the gas heated to tens of millions of degrees and the galaxies moving with typical velocities of approximately 1000 km/s. Because of their extreme environments, clusters are ideal places to study how galaxies are affected by their surroundings, namely through galaxy interactions and through the collision with the cluster gas. At the same time, by observing clusters when the Universe was half its present age the astronomers can see how the galaxies in them have changed with time.

The research group is leading a project to address these issues. The ESO Distant Cluster Survey (linkPfeil.gifEDisCS) consists of detailed observations of 20 clusters spread over a redshift range of 0.45 < z < 0.8. The observations were performed with two telescopes in Chile; 36 nights of observations were performed with the Very Large Telescope (linkPfeilExtern.gifVLT) on Paranal and 20 nights were performed with the New Technology Telescope (linkPfeilExtern.gifNTT) on La Silla.

One main result at this early stage is that the EDisCS clusters appear to have a large range in mass (see Figure 1) where the mass of the clusters has been directly measured using an effect called weak gravitational lensing. This effect measures correlations between the orientation of background galaxies that are caused when their light passes through the gravitational potential of the foreground cluster. The advantage of this technique is that it measures the total mass in each cluster, including the dark matter. While some of the clusters are among the most massive at their distance, some are have such a low mass that they cannot be detected with the weak gravitational lensing technique.

Fig. 2: This Figure demonstrates the incredible power of the Hubble Space Telescope (HST) with its new instrument, the Advanced Camera for Surveys. The images in the lower panel were taken in very good conditions from the Very Large Telescope in Chile. The images in the upper panel were taken with HST and have more than 10 times higher spatial resolution. There is a wealth of structure in the HST images which is simply not possible to see in the ground based images.

Fig. 3: Images of two clusters taken with the Hubble Space Telescope compared to the same clusters observed with the Very Large Telescope in Chile. Although the Very Large Telescope produces very deep images of excellent quality, the resolution is clearly surpassed by that of the Hubble Space Telescope. In the cluster cl1216-1201, the faint circular arcs seen in the image come from a distant background galaxy whose light is being strongly distorted by the shape of the cluster.

In addition to probing the properties of the clusters themselves the astronomers have been able to discern the morphologies (i.e. shapes) of these galaxies using deep images from the linkPfeilExtern.gifHubble Space Telescope using the newly installed Advanced Camera for Surveys. These images have more than 10 times the resolution of the excellent ground-based images and give a new window on the physical nature of the galaxies (See Fig. 2 and 3). Among the early results from the morphological studies is that there are many red disks in the EDisCS clusters, and that the morphological mix of galaxies changes greatly from cluster to cluster.

Gregory Rudnick

For Further Information:

Gregory Rudnick et al. linkPfeilExtern.gif"Studying High Redshift Galaxy Clusters with the ESO Distant Cluster Survey", The Messenger, 112, p. 19