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The history of the scientific investigation of galaxy clusters starts with the XVIII century, when Charles Messier and F. Wilhelm Herschel independently produced the first catalogues of nebulæ, and noticed remarkable concentrations of nebulæ on the sky. Many astronomers of the XIX and early XX century investigated the distribution of nebulæ in order to understand their relation to the local ``sidereal system'', the Milky Way. The question they were trying to answer was whether or not the nebulæ are external to our own galaxy. The answer came at the beginning of the XX century, mainly through the works of V.M. Slipher and E. Hubble (see, e.g., Smith [424]).

The extragalactic nature of nebulæ being established, astronomers started to consider clusters of galaxies as physical systems. The issue of how clusters form attracted the attention of K. Lundmark [287] as early as in 1927. Six years later, F. Zwicky [512] first estimated the mass of a galaxy cluster, thus establishing the need for dark matter. The role of clusters as laboratories for studying the evolution of galaxies was also soon realized (notably with the collisional stripping theory of Spitzer & Baade [430]).

In the 50's the investigation of galaxy clusters started to cover all aspects, from the distribution and properties of galaxies in clusters, to the existence of sub- and super-clustering, from the origin and evolution of clusters, to their dynamical status, and the nature of dark matter (or ``positive energy'', see e.g., Ambartsumian [29]). As a matter of fact, the topic expanded so much that in 1959 a new separate section specifically devoted to galaxy clusters - Galaxienhaufen - appeared in the Astronomischer Jahresbericht. Galaxy clusters had become one of the main research topics in extragalactic astrophysics.

In this historical review I have tried to cover all aspects of astrophysics research on galaxy clusters, spanning a temporal range of exactly 200 years, from 1784 to 1983. In 1784, Charles Messier [303] was the first to write about a cluster of galaxies, Virgo, in his Catalogue des nébuleuses et des amas d'étoiles que l'on découvre parmi les étoiles fixes, sur l'horizon de Paris. In 1983, on October 7th, George O. Abell, the eponymous of nearby rich clusters of galaxies, prematurely died at the age of 56. A practical reason for stopping this review with 1983, is that the exponential increase of publications makes it increasingly difficult for the historian to keep pace with the new scientific results.

This review is divided into four main topics:



  3. THE CLUSTER STRUCTURE, including:


I consider here both theoretical and observational aspects. However, I rarely mention technical aspects, such as the development of new telescopes and instruments, which were certainly very relevant to our understanding of galaxy clusters. In this respect, this review traces the history of the scientific thought, rather than the history of science.

For the sake of homogeneity, all quantities that are H0-dependent, have been re-scaled to the same value the Hubble constant, H0 = 75 km s-1 Mpc-1.

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