Next Contents Previous

". . . and it is lawful to call it a new world, because none o f these countries were known to our ancestors, and to all who hear about them they will be entirely new. "

Amerigo Vespucci, 1503

"Let Christ rejoice . . . in the prospect o f the salvation o f the souls of so many nations hitherto lost."

Christopher Columbus, 1493


This atlas represents almost the first attempt to map the structure of our environment on a scale significantly larger than the Milky Way Galaxy. The most substantial effort to precede ours was by de Vaucouleurs (ref. 11). Two decades have passed since the completion of that seminal work. Today, many more galaxies have been observed. Modern observatories in the Southern Hemisphere are providing information of comparable quality with observatories in the north, so there is now uniform coverage of the entire sky. It is time to plot the data and see what our corner of the Universe looks like.

It is a surprising sight, because there is so much organization. These maps are not the first evidence for large-scale structure (see references). Astronomers are beginning to,accept that the local part of the Universe is laced with filaments of galaxies. However, our impressions have been based on very sketchy information. It has been known that there is a significant enhancement of galaxies in a plane that is the main component of what has been called the Local Supercluster, but two important points had not been appreciated: one, that other nearby clouds of galaxies tend to align themselves parallel to this same plane, and two, that this plane is incredibly extensive.

It is evidence of the state of our ignorance that the preceding sentence remains controversial at the time this work is published. The main purpose in producing the atlas is to promote research regarding the nature and origin of large scale structure. However, the interpretation of an atlas is straight forward. Consequently, this work has been designed to appeal to a wide audience.

A number of assumptions must be made on the way to a representation of the complex three-dimensional structure found to exist. The distances of galaxies are still quite insecure. Our surveys are still incomplete. The procedures that have been adopted to address these and other problems are only briefly summarized and not defended in this text.

There are three groups of maps in the atlas. The first group portrays the observational material in a very direct way and does not require much explanation. All the galaxies in our sample are plotted in ten maps that cover the entire sky and there is a blow-up of a crowded region on an eleventh map. The sample consists of all galaxies that had a known velocity of less than 3,000 kilometers/ second in 1978. Because of the expansion of the Universe, there is a direct relationship between the velocity of a galaxy and its distance. The cutoff in velocity associated with this sample is equivalent to a cutoff in distance, so that the first group of maps shows the distribution on the sky of all known galaxies within a specified distance from us. The information contained in these maps can be reasonably trusted. In future cartographic efforts, more galaxies will have been discovered and added to the maps but those there now will not be lost.

The second group of maps is designed to illustrate the three-dimensional distribution of these same galaxies. There are uncertainties in the details of these maps. There are ambiguities in the link between galaxy velocities and distances. Incompletion is a problem. Future generations will smile at the simplicity of the present map renditions. They are a first step.

The third group of maps are an afterthought and not quite in keeping with the nearby in the title of the atlas. It can be concluded from what is seen on the maps of the second group that there must be structure on still larger scales. There are entities that fill substantial fractions of the volume we considered and it can be assumed that they continue beyond our boundaries. Unfortunately, there are no complete all-sky samples of galaxies with measured velocities that would allow us to probe the nature of structure on substantially larger scales. However, a reasonably complete sample of rich clusters of galaxies that extends to distances corresponding to a velocity of 30,000 kilometers/ second does exist, and their distribution can be studied in the same way the distribution of individual galaxies can be studied locally.

The maps of the distribution of rich clusters cover a significant fraction of the observable Universe. The light travel time from objects at the outer limits of these maps is a tenth of the age of the Universe. Hence, the light we see today from the most distant regions of our maps was emitted ten percent of the way back in time toward the moment of creation.

One aspect of this pioneering endeavor is that we get to invent nomenclature and name things. An attempt has been made to be consistent with current astronomical vocabulary, but some clarification is in order.

Enhancements in the density of galaxies on small scales are called clusters or groups. Random motions are sufficient that individual,galaxies can traverse the regions of clusters or groups in times less than the age of the Universe. An entity is defined as a cluster if members have random motions significantly higher than what they would be in an isolated environment. That is, a cluster is a system that has proceeded well along toward a relaxed state. Groups have low random motions, as though they are just forming, and consist of relatively small numbers of galaxies.

Enhancements in the density of galaxies on large scales can be called superclusters, clouds, filaments, or spurs. In each of these cases, the random motions of galaxies are insufficient for them to traverse the large-scale structure in the age of the Universe. The difference between a supercluster and a cloud is just one of scale. A supercluster is an enhancement in the density of galaxies on scales of order 100 million light years (30 megaparsecs), while clouds are density enhancements on scales of order 30 million light years (10 megaparsecs). A supercluster is made up of several clouds. Empty regions around superclusters and clouds help to delineate them. Frequently, clouds appear to string together in long, one-dimensional structures, and these we call filaments. Sometimes major clouds bifurcate, and we may refer to minor appendages as spurs. On very large scales, the rich clusters are observed to congregate in regions as large as 1 billion light years (300 megaparsecs) across. These immense regions are called Supercluster complexes.

These descriptive terms are helpful because they promote a familiarity with our large-scale environment. However, there is a continuum of scales and a substantial range of density enhancements in these structures. Even among the small number of features called clouds that are found in the small fraction of the Universe explored here, there are significant variations in properties.

A word should be said about the names given to features that are identified. In some cases, historical names have been retained; for example, the Southern Supercluster and the Virgo Southern Extension. In other cases, features that were already known to exist have been renamed in order to encourage a more coherent nomenclature. Perhaps the most audacious rechristening concerns the cloud that we live in. It has been called the Local Cloud (ref. 12), but we call it the Coma-Sculptor Cloud. Our name is derived from the fact that this feature extends from the constellation Coma Berenices to the constellation Sculptor on the plane of the sky. Those familiar with the constellations could guess that we must reside in this cloud because the two constellations are in diametrically opposite directions. In general, features are given the name of the single constellation that they are principally identified with, or of the two constellations that mark their extremities. An index of names is provided.

Next Contents Previous