Superclusters and voids

Now that we have seen that galaxies are gathered into clusters, we might ask if the clusters are gathered into larger groupings, called superclusters. The answer is that they are. The first supercluster identified (in the 1950s) is the one in which we live, called the local supercluster. The Virgo cluster of galaxies is near the center of the local supercluster. The local group, our cluster of galaxies, is near the edge. The local supercluster contains 106 galaxies in a volume of about 1023 cubic light years!

Studies of more distant superclusters have been made difficult by a lack of extensive data on distances to galaxies. After all, we only see two dimensions projected on the sky. We can get a better idea of clustering if we also know distances to galaxies. We obtain the distances from measuring the redshifts of clusters.

It might seem that we can determine the mass of a supercluster by using the virial theorem, in much the same way as we did for clusters of galaxies. However, the crossing time for a cluster in a supercluster is greater than the current age of the universe (as estimated by the Hubble time). This means that superclusters are not dynamically relaxed, and the virial theorem should not apply. It is even possible that superclusters are not gravitationally bound.

In addition to superclusters, there appear to be voids of comparable size. As their name implies, voids are large regions of space that are essentially devoid of galaxies.

One of the major breakthroughs in these studies has come from the ability to measure a large number of redshifts in a relatively short time. These redshift surveys are carried out at radio (21 cm) and optical (Ha) wavelengths. They cover large sections of the sky, and also cover a large range of redshifts. Having surveys in both the optical and radio parts of the spectrum provides an important check on the results.

Presenting the results of these redshift surveys can be difficult. Not only are there millions of data points, we have the location of each galaxy as a function of three coordinates - two for the position on the sky, and one for the redshift. We can leave the third coordinate in terms of red-shift, or convert it to a distance, using Hubble's law. In any case, we are still stuck with trying to plot a three-dimensional distribution of galaxies. One way of doing this is to make slices through our data, and then for each slice make a two-dimensional picture of the resulting distribution of galaxies. In Fig. 18.16, we show how we might uses slices through a three-dimensional Earth to produce a series of two-dimensional images, which, taken together, give us a feel for the three-dimensional structure. For the redshift surveys, we can make our slices showing distribution on the sky in a series of redshift ranges, or showing distribution in one sky coordinate and redshift for a range of the other sky coordinate.

Some representative survey results are shown in Fig. 18.17. In these figures, each dot represents a galaxy; a concentration of dots is a cluster; a

Fig 18.16.

How we might use slices through a three-dimensional object to let a series of two-dimensional images represent our three-dimensional structure.

Fig 18.16.

How we might use slices through a three-dimensional object to let a series of two-dimensional images represent our three-dimensional structure.

concentration of clusters is a supercluster, and a lack of dots is a void. From these figures, we see that superclusters and voids are quite common. There is a very distinctive feature to the distribution. It looks like the galaxies are concentrated on the surfaces of various shapes. The first astronomers studying these distributions thought that the surfaces may be like soap or beer foam bubbles, but it now seems that the structures are even more complicated. Some have made analogies with sponges and swiss cheese, with the typical sizes of the holes being tens to a hundred million parsecs across. This shows us that the galaxies are distributed in a much more complicated way than is implied by simply talking about super-clusters and voids.

From these surveys, we can also deduce the motions of the galaxies with respect to their neighbors. Remember, earlier in this chapter we saw that any galaxy has motion associated with the expansion of the universe, and local motions in response to the gravitational attraction of its neighbors. When we analyze these local motions of the galaxies, we find that they are are not completely random. They are organized, with galaxies in some part of a shell having similar motions. This suggests that there are large amounts of matter attracting the galaxies to produce the organized motion.

We can also see a similar effect in the motion of our galaxy (or the Local Group) through space. How do we detect that motion? Think of the following analogy. Imagine that you are in a room full of people, all standing still. You start to walk through the room. All of a sudden, it appears that people on one side of the room are coming towards you, while people on the other side of the room are moving away from you. Your first thought might be that this is the way that the people are actually moving. On farther thought, you realize that you are seeing your own motion reflected in the apparent motions of the people.

The situation is a little more complicated with galaxies, because the universe is expanding. We start with all of the galaxies moving away from us. If we then start moving in some direction, we will be overtaking some galaxies, and moving away from others in the opposite side of the sky. On average, galaxies in one half of the sky will appear to be moving away from us slightly faster than in the other half of the sky. This effect is actually observed. By seeing which half of the sky appears to be moving away from us a little faster, and which half is moving a little slower, we can determine how fast we are moving and in what direction. What we actually measure is the total motion of the Earth. We must then correct for the motion of the Earth around the Sun, the

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