Chapter_9 - Chapter 9 Clusters and Superclusters If the...

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Chapter 9 Clusters and Superclusters If the universe is a loaf of raisin bread, as Figure 7.17 suggests, then the raisins are not uniformly distributed through the loaf. Speaking less metaphorically, galaxies are not uniformly distributed through space. Figure 9.1 is a plot of the distribution of galaxies with m B < 19) in the northern sky. The plot Figure 9.1: Distribution of galaxies in the Northern Galactic Hemisphere; the missing slice at lower right is inaccessible from Mount Hamilton, CA, where Lick Observatory is located. [Image credit: Seldner, et al. 1977, AJ, 82, 249] 205
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206 CHAPTER 9. CLUSTERS AND SUPERCLUSTERS is based on the Shane-Wirtanen catalog of galaxies. Shane and Wirtanen photographed the portion of the sky visible from Lick Observatory, then spent long hours counting the 10 6 galaxies detected on the photographic plates. In the plot of the Shane-Wirtanen counts, we deFnitely see a non- uniform distribution. People tend to use the words “bubbly” or “spongy” when they describe the large-scale distribution of galaxies. The galaxies lie along walls or Flaments, with particularly strong concentrations in the clusters where Flaments meet. The universe shows hierarchical structure ; that is, it contains structure on a very wide range of length scales. Stars (typical diameter d 10 6 km) are found largely in gravitationally bound objects called galaxies, containing 10 6 10 12 stars, plus gas, dust, and dark matter. Galaxies (typical diameter d 10 kpc) are found largely in gravita- tionally bound objects called clusters, containing 10 10 4 galaxies, plus gas and dark matter. Clusters (typical diameter d 1 Mpc) are found largely in currently collapsing objects called superclusters . The superclusters are the largest structures you can see in ±igure 9.1, and have a maximum length d 100 Mpc. 9.1 Clusters of Galaxies To start at home, our own galaxy is part of a relatively small cluster called the Local Group . The Local Group, mapped in ±igure 9.2, contains at least 40 galaxies. The exact number of galaxies in the Local Group is not known, since most of the galaxies are inconspicuous dwarf spheroidal and dwarf irregular galaxies. The discovery of a new Local Group galaxy – a dwarf spheroidal given the name of Andromeda X – was announced as recently as January 2006. Like most small clusters, the Local Group is irregularly shaped. Most of the galaxies cluster around our own galaxy (labeled “Milky Way” in ±igure 9.2) and M31 (labeled “Andromeda” in ±igure 9.2). Our own galaxy and M31 contain most of the mass and luminosity in the Local Group. Third and fourth place go to M33 (an Sc galaxy) and the Large
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9.1. CLUSTERS OF GALAXIES 207 Figure 9.2: The Local Group, with galaxy types indicated; note the preva- lence of dwarf galaxies.
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Chapter_9 - Chapter 9 Clusters and Superclusters If the...

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