When we studied stars, we discovered that they were found in specific groupings that you might see as analogous to how people populate communities on Earth. For example, we see star clusters, which are in some ways like "cities" filled with about half a million stars. We also find stars in groupings of billions of stars inside of galaxies, so you might consider galaxies to be similar to "countries" of stars, although no country on Earth has 100 billion people!
Now that we are studying galaxies, we can use observations of the environment around the Milky Way to inform us of the types of communities in which galaxies are found.
The Local Group
If we survey the local universe around the Milky Way, we find that there are a few dozen galaxies in our neighborhood. More specifically, we define our “group” to include those galaxies that we believe to be mutually bound to each other by the force of gravity. Collectively, we refer to this group of galaxies that includes the Milky Way as the Local Group of galaxies. The Local Group contains only a few large galaxies, all of which are spiral galaxies. These are the Milky Way, Andromeda (M31) and M33, which is also called the Triangulum Galaxy.
Most of the rest of the Local Group members are different types of dwarf galaxies. There are irregular galaxies, like the Large Magellanic Cloud (LMC) and Small Magellanic Cloud (SMC), there are dwarf elliptical galaxies like M32, which is a satellite of Andromeda, and there are dwarf spheroidal galaxies like Sculptor, Fornax, and Ursa Minor (named after the constellations in which they are found). For the most part, though, these objects are all much smaller and less massive than the Milky Way and Andromeda. If we compare the galaxies by their luminosities, for example, the Large Magellanic Cloud is about 1/10th as bright as the Milky Way, which suggests that it has about 10% of the mass of the Milky Way. Some of the smallest dwarf spheroidal galaxies in the Local Group are as faint as 1/50,000th the brightness of the Milky Way. Therefore, even though the dwarf galaxies are dominant in number in the Local Group, the three spirals dominate the mass of the Local Group. This is common in most environments. There are a range of galaxy sizes and masses, and there are fewer of the most massive galaxies and many more of the dwarf galaxies.
The galaxies of the Local Group are distributed within a volume of about 1 Megaparsec (Mpc) in radius. The volume of a sphere with a radius of 1 Mpc is about 4 cubic Mpc. This means that there are about 10 galaxies per cubic Mpc in the Local Group.
If you move outward from the Local Group, you encounter several other groups that are superficially similar to our own. M81 and M82 are two of the largest members of their group about 4 Mpc away from us, which also includes a total of about two dozen known galaxies. Centaurus A is a member of the M83 group, which is about 5 Mpc away. The Local Group, the M81 group, and the M83 group are examples of what are generally called “loose groups,” which are just what they sound like: bound groups of galaxies that are not very densely packed together.
You have already seen images of the Milky Way Galaxy from the Digital Universe Atlas, where the disk of the Milky Way was represented as a flat image, and the dark matter halo of the Milky Way was represented with a wire frame sphere. Below is a short movie, captured from the Digital Universe Atlas, showing the point of view of an observer flying around the Milky Way. The green dots that you see represent the 3D positions of the galaxies in the Local Group. In this view, you see primarily the satellites of the Milky Way (e.g., Fornax, Leo I, Leo II, Sculptor) because Andromeda, M33, and their satellites are more distant (in fact, they are the dense group of green dots you see at the edge of the field of view of the movie).
Try this with Starry Night!
For much of this unit, we are studying objects that we simply are unable to see by casually observing the night sky with the unaided eye. However, there is one prominent exception. If you happen to live in or visit the southern hemisphere, you can see two Local Group galaxies without a telescope. The Large and Small Magellanic Clouds are visible as faint, fuzzy patches of sky distinctly separate from the plane of the Milky Way. If you use Starry Night to change your location to Sydney, Australia, you should be able to find the Clouds in the night sky.
The Virgo Cluster
About 17 Mpc away from the Milky Way Galaxy is a much larger group of galaxies. This group is found inside of the constellation Virgo, and it is usually called the Virgo Cluster. The Virgo Cluster is not much larger than the Local Group (about 3 Mpc in radius), but it contains approximately 2,500 galaxies. If you calculate the volume of a sphere 3 Mpc in radius, you will find that there are only about 25 galaxies per cubic Mpc in Virgo. However, like the stars in a globular cluster, the galaxies inside the Virgo Cluster are very densely packed in the inner region of the cluster, and the density drops off as you get farther from the center. So, the conditions in the core of the Virgo Cluster are very different from those in the Local Group. Actually, clusters show a range of central concentrations, and Virgo is a pretty bad example to use here, since it is one of the least concentrated of galaxy clusters.
The galaxy M87 is the dominant member of the Virgo Cluster. It is a large, massive elliptical galaxy and it also harbors an AGN that is creating a powerful jet and lobes of radio emission. If you take a look at the wide-field image of the Virgo Cluster above, you will notice that M87 is very easy to pick out by eye. It is very obviously the largest galaxy in the image. Many clusters contain a dominant, giant elliptical galaxy, and these are called “cD” galaxies. In general, the types of galaxies found in clusters are different from the types found in groups. Clusters tend to contain a higher percentage of ellipticals, while the lowest density environments tend to contain more spirals.
Clusters like Virgo are enormous objects. If you consider that these gravitationally bound groups contain usually about 1,000 or more galaxies, that means their mass is about 1015 solar masses! They are considered to be the largest gravitationally bound objects in the universe.
By studying the velocities of galaxies inside of clusters like Virgo, we also find that galaxy clusters must contain dark matter (over and above what is in the galaxies themselves). X-ray observations of galaxy clusters show that they contain a large amount of very hot (millions of kelvin) gas, but even adding the mass of this gas to the mass in the visible galaxies cannot account for all of the mass needed to keep these clusters gravitationally bound.
In the neighborhood around the Milky Way, the local Universe contains a few small groups of galaxies and a cluster. As we continue to survey out from the Milky Way, we find that these types of structures are common throughout the Universe. Single galaxies are not found in complete isolation, without any companions in their neighborhood.