a) Do galactic clusters as a rule combine to form systems of higher order, such as superclusters or a supergalaxy? Our Local Group is evidently a member of a certain group of clusters whose center is represented by the large cluster in Virgo. This extensive spatial grouping was designated the Supergalaxy by de Vaucouleurs. It measures about 20 million parsecs across. However, we have as yet no evidence for the dynamic unity of this system, or for the existence of any forces capable of sustaining it.
At the same time, it is noteworthy that a study of the distribution of galaxies on the celestial sphere does not clearly reveal any appreciable number of such supergalaxies. When considering this problem, however, two conceivable possibilities should be taken into account: 1) that the distances between supergalaxies are very large in comparison with their diameters; and 2) that these distances are of the same order as the diameters of the supergalaxies.
In the first case, many of these supergalaxies would be distinctly visible against the celestial sphere as isolated formations. In the second case, we would see projected against the sky only a small number of relatively near isolated systems of this kind, and by means of a superficial study it would be difficult to deduce the existence of any distant super galaxies.
Observations show clearly that the distribution of galactic clusters and groups is nonuniform, and the existence of supergalaxies may explain this to a certain extent. It may be assumed at the same time that we only observe a few isolated nearby clouds consisting of a large number of condensations. The only thing which has been definitely established is the presence of a large cloud in the southern sky, extending between galactic longitudes of 160° and 240° and having a galactic latitude of -40°.* These two facts indicate that the second alternative is the correct one, that is, that supergalaxies do exist and that the distances between them are of the same order as their diameters.
Thus, even though the existence of individual supergalaxies seems to be an established fact, there are still certain questions which remain unanswered. For example, it is not known what percentage of the galactic clusters are members of such higher-order systems, or whether both of the known types of clusters (globular and diffuse) exhibit an equally strong tendency toward bunching together. Answers to these questions will be possible only on the basis of more detailed photometric and statistical studies.
b) To what extent do low-luminosity galaxies have the same spatial distribution as high-luminosity galaxies? As we said before, the fact that galaxies concentrate in clusters has been quite well established for high luminosity objects. Objects of low luminosity, however, lying at distances of a few million parsecs or more, would be completely lost among the galaxies of the distant background, so that the question is not very easy to settle in this case. On the other hand, galaxies with low surface
luminosities are one class of low-luminosity objects about which something is known. It has been found by Rijves that the distribution of objects with low surface luminosities in the Virgo cluster is roughly the same as the distribution of high-luminosity galaxies. On the other hand, it cannot yet be established with any certainty whether galaxies at the low limit of surface luminosity (such as the Sculptor system or the Zwicky object in Capricornus) constitute a general metagalactic field or whether they are concentrated in clusters and groups.
c) The supergalaxies discussed above are objects with diameters of the order of 20 million parsecs. If these represent the largest inhomogeneities in the distribution of galaxies, then any given portion of space measuring 50 or 100 million parsecs across can be expected to have approximately the same amount of matter (galaxies) in it. It is possible, however, that inhomogeneities of a larger scale exist. Whether this is true or not can be decided only by studying the distribution of faint galactic clusters (up to the 21st stellar magnitude), or by studying the distribution of extra galactic radio sources. The solution of this problem is very important, since it will provide a check of the various cosmological theories proposed. At present it can only be stated that no justification for the postulate of homogeneity usually adopted by cosmologists has yet been found.
d) It was mentioned previously that there is strong evidence for the existence of intergalactic dust matter. In this connection, a study of all the various kinds of intergalactic matter would be advisable. So far, the following kinds have been identified: 1) Luminous intergalactic matter, which sometimes fills the central part of the volume occupied by a galactic cluster. All the data indicate that this luminous matter consists of stars, in the same way as the bridges and bars often observed in binary galaxies do.
2) Intergalactic globular clusters, a few of which have been detected at distances of more than 100,000 parsecs.
3) Giant clouds of relativistic electrons which have been ejected from the interiors of galaxies. Radio source Centaurus A, for instance, consists of three such clouds, while radio source Cygnus A consists of two. Each cloud of this type is larger than a normal-sized galaxy, and many such clouds must have been scattered throughout intergalactic space in the past.
4) Absorbing dust. There do not seem to be any data available on the sizes of individual dust clouds.
5) Neutral gas masses, which seem to be present in such small amounts that the radiation they emit (for instance, in the x = 21 cm line) has not yet been detected with any certainty.
Each of the foregoing kinds of intergalactic matter definitely deserves a special study.