1.1. Binary Systems Among Stars and Galaxies
The systematic investigation of double galaxies provides us with important information about the conditions of formation and properties of the evolution of galaxies as stellar systems. We make the basic assumption that galaxies in isolated pairs move in bound orbits, which probe their mutual gravitational influence, just as for double stars. Also, as for these stars, random velocities of approach among galaxies are extremely rare, and therefore we may conclude with high probability that galaxies and double systems form together and that their subsequent evolution proceeds at parallel rates.
According to this notion, galaxies in double and multiple systems were so formed in the early stages of the evolution of the Universe when the mutual separation of galaxies (or protogalaxies) was comparable to their individual dimensions. The great rarity of chance encounters among galaxies causes some original features to be `frozen' into systems of galaxies after their formation, which means that we study the conditions of formation of binary galaxies from their observed properties at the present epoch.
Furthermore, there exist many very close pairs in which galaxies are in close contact, or overlapping each other. The components of these contact systems exhibit such strong mutual influences that the resultant distortions of the galaxy structure merge eventually into a single stellar system. Therefore, close pairs (often with very clear signs of interaction) are unique objects for studying the synchronized evolution of stellar systems.
Continuing the analogy with double stars, we note that the study of their physical properties and distribution has proved very fruitful for constructing models of stellar evolution. [add references] In several regards, observations of double stars give the most direct and therefore the most reliable data on the basic characteristics of stars. For example, consider the determination of the masses of the components of double stars. In fact, the investigation of double stars has been one of the central activities of stellar astronomy, as shown by numerous monographs and textbooks (Agekyan, 1962 and Martin, 1971). [add references]
In the same literature, we find no detailed description of the observational properties of double galaxies. There are several reasons for this:
In contrast to the situation for stars, the radial velocities of double galaxies have additional, individual information content. According to the Hubble Law of proportionality between radial velocity and distance, we may take the radial velocity of a pair as a measure of its distance from the observer, thus allowing calculation of the absolute properties of the galaxy from the observed ones, such as apparent magnitude or angular diameter. Thus, in recent decades, much of the observational effort on double galaxies has concentrated on new measures of radial velocities.
A separate and growing interest in double galaxies comes from the question of measuring the masses of galaxies. At the basis of our knowledge of galaxy masses lie measurements of the motions of stars and gas in these systems. Assuming circular motions, we can also calculate the total masses of galaxies according to particular models based on optical and radio observations. Unfortunately, the various estimates of the mass do not show satisfactory agreement. The most distant regions of galaxies are difficult targets for spectroscopic observation, since the surface brightness is only a few percent of that of the night sky. Measurement of the rotation of galaxies using the radio HI 21-cm line can sometimes continue on to even greater distances from the center of the galaxy, but these data concern the rotation of only the gaseous component of the objects. In the not improbable case that the gas motion departs from circularity, the resulting estimates of total mass will be subject to systematic errors. There has long been an unresolved debate about whether or not there exist massive halos, or coronae, with a mass possibly surpassing the total stellar mass. [add references]
The proper conclusion seems to be that different methods for estimating the masses of galaxies are subject to various substantial errors. Calculational methods for group members and cluster members from the virial theorem give masses an order of magnitude larger than those measured from rotation curves. This `virial paradox' remains one of the paramount questions of extragalactic astronomy. Its solution may remove the main uncertainties in cosmology, by providing the mean mass density of the Universe which gives rise to the metric properties of the Universe as a whole. [add references]
By comparison with virial mass estimates, which incorporate basic assumptions about the statistical balance between kinetic and potential energy of the members of a system, the calculated mass of double galaxies rests upon much simpler assumptions about closed Keplerian motions of galaxies acting as point masses. For such cases, studying a distant enough sample of pairs measures not only the outer regions but also any unseen matter in the space between the pair members. We then have the apparently promising possibility of comparing orbital motions to measures of mass from rotation curves in parts of the galaxies. Therefore, the growth in recent years in the number of measured radial velocities for double galaxies is an important step not only in studying these systems as such, but also in the fundamental role of measuring the total masses of galaxies.