intergalactic medium

the gas, radiation, cosmic rays, magnetic fields, neutrinos, and other material found in the space between the galaxies. Most often, the intergalactic medium is not directly observed, and its existence and properties are deduced from indirect data. Intergalactic gas, according to theoretical research, is the remnant of the matter from which the galaxies were formed in the past. Conclusions about the density of the intergalactic medium are of great cosmogonic significance: the course of the subsequent evolution of the universe is connected with the average density of the matter in it.

An upper limit for the density of the intergalactic medium is inferred from the absence in the spectra of celestial bodies of certain emission and absorption lines, which are caused by physical processes in the intergalactic medium and would be observed under other conditions. Thus, the absence of absorption at the 21-cm line enables a limiting value to be computed for the density of neutral hydrogen at relatively small distances from our galaxy. A better estimate can be obtained by analyzing the reasons for the absence of the La absorption line in the spectra of distant quasars (p < 10^-35 g·cm^-3); this estimate relates to great distances, and, consequently, to a time when the universe was three to four times younger than it is now. The low density of hydrogen atoms indicates that the gas at that time was strongly ionized and, apparently, had a temperature of the order of 10⁵ °K.

Inferences about the density of the intergalactic medium may also be made on the basis of a study of its accretion (capture) by clusters of galaxies, the motion of radio-emitting ejected matter, and other indirect data. The presence of gas within certain clusters of galaxies is assumed in order to explain the stability of such clusters. From a theoretical point of view, it is impossible to exclude the possibility that a substantial portion of the intergalactic medium consists of low-energy neutrinos, which are impossible to detect with existing methods.

The intergalactic medium was evidently heated by cosmic rays and X-radiation from radio galaxies, quasars, and newly forming galaxies. Shock waves spreading out from condensing gas masses during the formation of clusters of galaxies may also have been important. lonization of the hot gas may have been supported by ultraviolet radiation and X-radiation from various young objects. The temperature and degree of ionization of the gas have apparently decreased with time.

Radiation in the intergalactic medium includes the isotropic relict background radiation with a temperature of about 3°K, as well as radiation from galaxies, quasars, and the intergalactic medium itself in all wavelength regions. The latter permits an estimate of the upper limit of the density of ionized hydrogen. Data on cosmic rays and the magnetic field in the intergalactic medium are still very uncertain, but it is known that relativistic electrons (electrons moving with velocities close to the velocity of light) are significantly scarcer in the intergalactic medium than in our galaxy, since otherwise they would produce observable radiation in interactions with the photons of the intergalactic medium. At present, it is impossible to estimate the density of metagalactic neutrinos.