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Forty years after the discovery that galaxies were independent stellar systems, we still have not penetrated very far into the mystery of how they maintain themselves or what physical forces are responsible for shaping their observed forms. The galaxies are the constituent units of mass and energy in the universe, and yet we are still challenged by such questions as: What causes the characteristic shape of spiral galaxies? How are elliptical galaxies related to spiral? How are galaxies formed, and how do they evolve?

It is difficult to resist an oversimplified impression of what a galaxy is because the Hubble classification divides the galaxies into the well-known categories of smooth, amorphous ellipticals and flattened spirals with star-studded arms. But not all galaxies fit the schematic idealization of the Hubble sequence of nebular forms. In fact, when looked at closely enough, every galaxy is peculiar. Appreciation of these peculiarities is important in order to build a realistic picture of what galaxies are really like.

But the peculiarities are also important for another reason. If we could analyze a galaxy in the laboratory, we would deform it, shock it, probe it in order to discover its properties. The peculiarities of the galaxies pictured in this Atlas represent perturbations, deformations, and interactions which should enable us to analyze the nature of the real galaxies which we observe and which are too remote to experiment on directly. In general, the more conspicuous the peculiarity, the more illustrative it is of special events and reactions that occur in galaxies. Therefore the greatest deviations from the normal are emphasized in this Atlas. In some cases small peculiarities are included to illustrate, in sequence, how a certain type of peculiarity develops in importance until it dominates the form of the object. But it is from this overall range of experiments that we must then select and study the ones which will give the most insight into the composition and structure of a galaxy and the forces that govern it.

The present Atlas specifically started from an attempt to better understand spiral galaxies. Many analyses, often complex mathematical treatments have been made over the years, starting from the assumption that spiral arms were the result of tracks of stellar orbits moving under the gravitational influence of a central force field. I believe that the forms of spiral arms, their bifurcations and convolutions cannot be explained by such theories. In 1962 I undertook to assemble a series of photographs that would demonstrate this point. In the investigation of these spiral properties, galaxies which showed unusual or perturbed arms or filamentary extensions were sampled with high-resolution photographs with the Palomar 200-inch telescope. Subjects were first drawn from the pioneering work on peculiar galaxies by Zwicky and Vorontsov-Velyaminov. So many important objects emerged under high-resolution, limiting-magnitude study, however, that the investigation into the nature of spiral arms was postponed in order to systematically organize these new phenomena into groups and to publish a representative sample of the most significant objects.

The Atlas as it has been realized in the following pages illustrates again that galaxies contain more than just stars, radiation, and gravitation. The pictures emphasize the importance of dust in some, they particularly imply a much more important role for the gas in general, and point to the existence of either new forces or forces which previously have been little considered. For example, if we consider galaxies to be merely an assemblage of mass particles, we should be able to treat them, in the limit, hydrodynamically as a frictionless fluid. But the twisted, distorted shapes and curious linkages pictured there suggest that viscosity-like forces are present. Dynamical friction does not seem sufficient because some of the filaments suggest a degree of viscosity approaching that of an elastic medium. Probably the only agency likely to account for this is that of magnetic fields that interconnect regions of wholly or partially ionized gas. Vorontsov-Velyaminov has stressed in the past the probable magnetic nature of some of the effects in peculiar galaxies.

Magnetic forces are very difficult to study optically, but are undoubtedly of great importance in our universe. Recent radio astronomy discoveries of violent events in galaxies reveal sources of energetic charged particles. These charged particles interact with magnetic fields and offer the hope of mapping, measuring, and understanding cosmic magnetic fields. The connection between the plasmas observed with the radio telescopes and the optical evidences of plasma effects pictured in the present Atlas is now open to us.

The overall aim of this Atlas is to present a number of examples of various kinds of peculiar galaxies. They are displayed in groupings that appear roughly similar, thereby furnishing also a rough, initial classification. Phenomena which each group represent may then be investigated by selecting the most favorable members in size or brightness, studying different members of the group in different orientations, and, finally, making some preliminary statistics of certain kinds of phenomena and their relationship to other observable parameters. It is hoped that this investigative procedure will not only clarify the workings of galaxies themselves, but reveal physical processes and how they operate in galaxies, and ultimately furnish a better understanding of the workings of the universe as a whole.

It is a pleasure to acknowledge the help of William Miller who photographically copied the original glass negatives, Lowell Peterson of Graphic Arts at the California Institute of Technology who supervised the large-size photographic reproduction of the Atlas, Frank Brueckel who carried out many computational tasks connected with the Atlas, and, of course, all those astronomers who suggested candidates for the Atlas from their own personal store of knowledge and who gave advice and encouragement.

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