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Our understanding of the structure of the Universe is based on the distribution of galaxies. Until the mid-1970s the number of galaxies with known distances (redshifts) was very small, thus conclusions on the structure were based on counts of galaxies. The largest of such counts was compiled in Lick Observatory by Shane & Virtanen [51]. This catalogue was analyzed by Seldner [49] and played a crucial role in the development of the hierarchical clustering scenario of structure formation by Peebles [42].

A big step in the study of the clustering of galaxies and clusters of galaxies was made by visual inspection of the Palomar Observatory Sky Survey plates with the aim to produce catalogues of galaxies and clusters of galaxies. The first of these catalogues was prepared by Abell [1] for clusters of galaxies. This catalogue covers the sky north of declination -27°. Abell, Corwin & Olowin [2] extended the cluster catalogue to the southern sky. Both these catalogues together contain 4074 clusters. A much larger catalogue was compiled by Zwicky [63]; in this catalogue all galaxies brighter than photographic magnitude mph appeq 15.7 as well as clusters of galaxies north of declination -2.5° are listed. Abell and Zwicky used rather different definitions of clusters. Abell clusters contain at least 30 galaxies in a magnitude interval of Deltam = 2, starting from the third brightest galaxy, and located within a radius of 1.5 h-1 Mpc (we use in this paper the Hubble constant in units H0 = 100 h km s-1 Mpc-1). Distances of clusters were estimated on the basis of the brightness of the 10th brightest galaxy. Clusters were divided to richness and distance classes. Zwicky used a more relaxed cluster definition, with at least 50 galaxies in a magnitude interval of Deltam = 3, starting from the brightest galaxy, located within a contour where the surface density of galaxies exceeded a certain threshold. Due to these differences some Zwicky clusters are actually central parts of superclusters which contain several Abell clusters and groups of galaxies (an example is the Perseus cluster). Since the definition of clusters in the Abell catalogue is more exact, this catalogue has served for a large number of studies of the structure of the Universe. On the other hand, the Zwicky catalogue of galaxies was the basic source of targets for redshift determinations.

An early catalogue of bright galaxies was compiled by Shapley & Ames [53]. Sandage & Tammann [48] published a revised version of this catalogue; it contains data on galaxies brighter than 13.5 magnitude, including redshifts. This catalogue, and the compilation of all available data on bright galaxies by de Vaucouleurs, de Vaucouleurs, & Corwin [18] were the sources of distances which allowed to obtain the first 3-dimensional distributions of galaxies. Much more detailed information on the spatial distribution of galaxies was obtained on the basis of redshifts, measured at the Harvard Center for Astrophysics (CfA) for all Zwicky galaxies brighter than mph = 14.5. Later this survey was extended to galaxies brighter than mph = 15.5 (the second CfA catalogue), and to galaxies of the southern sky (Southern Sky Redshift Survey) [14].

These early redshift compilations made it possible to discover the filamentary distribution of galaxies and clusters forming huge superclusters, as well as the absence of galaxies between them. These results were first reported in the IAU Symposium on Large-Scale Structure of the Universe [31, 55, 57, 58] and demonstrated that the pancake scenario of structure formation by Zeldovich [60, 61] fits observations better than the hierarchical clustering scenario. More detailed studies of the structure formation by numerical simulations showed that the original pancake scenario by Zeldovich also has weak points - there is no fine structure in large voids between superclusters observed in the real Universe [62] and the structure forms too late [16], thus a new scenario of structure formation was suggested based on the dominating role of the cold dark matter in structure evolution [5]. In a sense the new scenario is a hybrid between the original Peebles and Zeldovich scenarios: structure forms by hierarchical clustering of small structures within large filamentary structures - superclusters.

The next big step in the study of the large-scale distribution of galaxies was made on the basis of the catalogue of galaxies formed on the basis of digitized images of the ESO Sky Survey plates using the Automated Plate Measuring (APM) Facility [36, 37]. The APM galaxy catalogue covers 185 ESO fields, is complete up to magnitude bj = 20.5, and was the basis for a catalogue of clusters prepared by Dalton [15]. The analysis of the APM galaxy sample showed that properties of the distribution of galaxies differ from the standard CDM model which assumed that the density of matter is equal to the critical density. A low-density model with cosmological term (dark energy) fits the data better [19].

The modern era of galaxy redshift catalogues started with the Las Campanas Redshift Survey (LCRS). Here, for the first time, multi-object spectrographs were used to measure simultaneously redshifts of 50 - 120 galaxies [52]. The LCRS covers 6 slices of size 1.5 × 80 degrees, the total number of galaxies with redshifts is ~ 26, 000, and the limiting magnitude is bj = 18.8. Presently several very large programs are under way to investigate the distribution of galaxies in a much larger volume. The largest project is the Sloan Digital Sky Survey (SDSS), a cooperative effort of several North-American institutions with participants from Japan [34]. This survey covers the whole northern sky and a strip in the southern sky. The sky is first imaged in five photometric bands to a limiting magnitude about 23 (the limit varies with spectral bands), thereafter redshifts are measured for all galaxies up to a magnitude ~ 18, and active galactic nuclei (AGN) up to ~ 19; additionally a volume-limited sample of redshifts of bright elliptical galaxies is formed. The total number of galaxies with measured redshifts will probably exceed one million. Another large redshift survey uses the 2-degree-Field [35] spectrograph of the Anglo-Australian Telescope. This survey is based on the APM galaxy catalogue and covers two large areas of size 75° × 12.5° and 65° × 7.5° with limiting magnitude bj approx 19.5. The goal is to measure about 250,000 redshifts. It is expected that new redshift surveys give us the possibility to investigate the detailed structure of the Universe up to a distance of approx 2000 h-1 Mpc.

The largest systems of galaxies are superclusters, which are defined as the largest systems of galaxies and clusters still isolated from each other. Catalogues of superclusters have been constructed using Abell clusters of galaxies. The latest compilation by Einasto [26] contains 220 superclusters with at least two member clusters.

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