Galaxy Clusters and Superclusters
Galaxy Clusters and Superclusters
Goals: Observe the distribution of galaxy clusters and superclusters
Before starting, turn on: galaxy, Tully
You will be using: Abell, see command, censize command, 1Gly
Let's begin this tutorial by looking at the entire Tully galaxy catalog. Fly outside the Tully data set so you can see the entire cube; you may need to brighten up the data set.
Now, type the command
see abellto remove all the Tully galaxies except those inside dense clusters. These clusters are called Abell clusters after George Abell (1927-1983), who first cataloged them. Now you are left with a few dozen galaxy clusters that contain red-colored galaxies. Turn up the center size using the Censize Slider so that we know where the Milky Way is.
Turn on the Abell data group. Each point in this data group represents a cluster of galaxies. Some points may represent tens of galaxies, others may stand in for hundreds or even thousands of galaxies.
The first thing you should do is turn off the labels, you won't need them yet. Notice that those red clusters from Tully now are accompanied by a single point in Abell. Pull out to see the entire data set and turn on the 1Gly grid for scale.
As you orbit the Milky Way, you will see immediately that, unlike Tully, the Abell clusters do not readily show large-scale structure. In fact, the galaxy clusters do not appear to cluster or form connecting filaments with one another. However, some of the Abell clusters are grouped, and these groupings are indicated by the various non-gray points and the labels (turn on the labels now).
The color of the points represents their membership in superclusters. A supercluster is a cluster of galaxy clusters. By now, you know that the Virgo Cluster is the nearest galaxy cluster to the Milky Way. It's about 60 million light-years away, 15 million light-years in diameter, and contains over 1,000 galaxies. A supercluster contains many galaxy clusters and is on the order of 300 million light-years across.
Those galaxy clusters with a color other than gray are considered to be members of a supercluster. Let's filter the data set to see these clusters on their own. Abell has a data variable attached to it called notable. If notable = 1, then the galaxy cluster is part of a supercluster. So, use the only= command to see these notable galaxy clusters by typing
only= notable 1(note there is no space between the word “only” and the equal sign.)
Now you see the largest superclusters around our Local Supercluster, which is also called the Virgo Supercluster. Most of these superclusters are named for the constellations they appear in. Our supercluster contains the mighty Virgo Cluster and many other smaller groups such as the Ursa Major Group, the M51 Group, the M81 Group, and our group, the Local Group, among others. Now you see similar systems in the Universe, representing at least hundreds of thousands of galaxies.
Bring back all the Abell clusters by typing see all. The gray points, which are the majority of points, are not connected to any major supercluster, but remain as a nexus between the filaments and galaxies that must be present, but remain invisible in these data sets.
Next, we explore two partial galaxy surveys, the Two Degree Field Survey and the Sloan Digital Sky Survey.
© 2002-2005 American Museum of Natural History
Last Modified: 2007-12-19 by Brian Abbott