Distribution Of Galaxies

In the 1970s and 1980s, scientists at the Harvard-Smithsonian Center for Astrophysics measured the distances to over 30,000 galaxies in selected sectors of the sky. Distances were calculated from red shift data using Hubble's relation. The plan was to construct a three-dimensional map of the universe out to several hundred light-years. Leadership in this venture was provided first by Marc Davis (1947-) and later by John Huchra (1948—), Margaret Geller (1947-), and Valerie de Lapparent. The CFA surveys came as a major surprise. Instead of being distributed more or less uniformly in space, galaxies lie along long sheets and walls that surround large voids. The universe possesses a soap-bubble structure characterized by considerable local unevenness in the distribution of galaxies. Beginning with the uniform conditions indicated by the cosmic background radiation, the universe has evolved into a rather lumpy place—the task of explaining this fact in terms of relativistic models of galaxy formation in an expanding universe has not proved an altogether easy one.

During the 1980s a group of seven astronomers embarked on an international collaborative project to investigate a particular class of galaxies, the elliptical galaxies. The latter are ones that lack spiral arms, are free of gas, and are predominantly oval or round in shape. The astronomers, led by Sandra Faber (1944—) of the Lick Observatory and Trevor Lynden-Bell (1935—) of Cambridge University, were interested in the problem of galactic evolution and concentrated on the ellipticals because they are, as a class, quite uniform in their properties. To map the galaxies, they measured their red shifts and used Hubble's relation to estimate their distances. In the course of their investigation they derived an indicator that appeared to correlate very well with absolute luminosity. The indicator was the velocity dispersion of the galaxy, a quantity that measures the spread of the velocities of stars in the galaxy. Its correlation with luminosity allowed a measure of distance that could be used independently of the Hubble relation. Much to their surprise, the astronomers discovered that the galaxies they were studying possessed large and systematic "peculiar motions," velocities independent of universal expansion arising from the gravitational attraction of neighboring galaxies and matter.

The Milky Way galaxy is part of a larger collection of galaxies known as the Local Group. The Local Group in turn belongs to a system of clusters known as the Local Supercluster. The data of Faber, Lynden-Bell, and their associates indicated that the Local Group and the Supercluster as well as several other clusters are streaming toward a more distant concentration of mass. Dubbed the "Great Attractor" in 1986 by Alan Dressler (1948—), one of the investigators in the project, this mass consists of a large swell in the density of matter arising from a concentration of galaxies and "dark matter" in the direction of the southern constellations Hydra and Centaurus. The existence of the Great Attractor provided further evidence of local inhomogeneity in the distribution of matter in the universe.

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