12.2 The Distribution of Galaxies in Space

The region of the universe for which we have the most detailed information is, as you would expect, our own local neighbourhood. It turns out that the Milky Way Galaxy is a member of a small group of galaxies called, not too imaginatively, the Local Group. It is spread over about 3 million light-years and contains more than 54 members. There are three large spiral galaxies  (our own, the Andromeda galaxy, and M33), two intermediate ellipticals, and many dwarf ellipticals and irregular galaxies.

Several new dwarf galaxies have also been found near the Andromeda galaxy. Such dwarf galaxies are difficult to find because they typically contain relatively few stars, and it is hard to distinguish them from the foreground stars in our own Milky Way.

Figure 12.8 is a rough sketch showing where the brighter members of the Local Group are located. The average of the motions of all the galaxies in the Local Group indicates that its total mass is about 4 × 1012MSun, and at least half of this mass is contained in the two giant spirals—the Andromeda galaxy and the Milky Way Galaxy. And bear in mind that a substantial amount of the mass in the Local Group is in the form of dark matter.

Local Group

Illustration of the Local Group of Galaxies. The upper portion of the figure shows galaxies surrounding the Milky Way (center) out to “950,000 light years”. Labeled from top to bottom in the figure are: “And II”, “Ursa Minor”, “Leo II”, “Draco”, “Leo I”, “IC 1613”, “Sextans”, “LMC”, “SMC”, “Sculptor”, “Carina” and “Fornax”. The lower portion shows the galaxies out to “5.5 million light years”. Labeled from top to bottom are: “UGC-A86”, “The Andromeda Galaxy”, “Leo A”, “NCG 205”, “IC 10”, “M32”, “M33”, “NGC 185”, “NGC 147”, “Pegasus”, “Tucana”, “GR 8”, “NGC 6822”, “WLM” and “IC 5152”. The galaxy types are colour coded as yellow for “Spiral”, blue for “Elliptical” and red for “Irregular”.
Figure 12.8. This illustration shows some members of the Local Group of galaxies, with our Milky Way at the centre. The exploded view at the top shows the region closest to the Milky Way and fits into the bigger view at the bottom as shown by the dashed lines. The three largest galaxies among the three dozen or so members of the Local Group are all spirals; the others are small irregular galaxies and dwarf ellipticals. A number of new members of the group have been found since this map was made.

Small galaxy groups like ours are hard to notice at larger distances. However, there are much more substantial groups called galaxy clusters that are easier to spot even many millions of light-years away. Such clusters are described as poor or rich depending on how many galaxies they contain. Rich clusters have thousands or even tens of thousands of galaxies, although many of the galaxies are quite faint and hard to detect.

The nearest moderately rich galaxy cluster is called the Virgo Cluster, after the constellation in which it is seen. It is about 50 million light-years away and contains thousands of members, of which a few are shown in Figure 12.9. The giant elliptical (and very active) galaxy M87 belongs to the Virgo Cluster.

Central Region of the Virgo Cluster

Central Region of the Virgo Cluster. This image is dominated by the giant elliptical galaxy M87 at center. Hundreds of smaller, fainter galaxies seem to swarm around M87.
Figure 12.9. Virgo is the nearest rich cluster and is at a distance of about 50 million light-years. It contains hundreds of bright galaxies. In this picture you can see only the central part of the cluster, including the giant elliptical galaxy M87, just below centre. Other spirals and ellipticals are visible; the two galaxies to the top right are known as “The Eyes.”
Giant Galaxy is Still Growing by Chris Mihos (Case Western Reserve University)/ESO, CC BY 4.0.

Attribution

“28.3 The Distribution of Galaxies in Space” from Douglas College Astronomy 1105 by Douglas College Department of Physics and Astronomy, is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted. Adapted from Astronomy 2e.

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Fanshawe College Astronomy Copyright © 2023 by Dr. Iftekhar Haque is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.