intergalactic medium

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Related to intergalactic medium: intergalactic matter

intergalactic medium

(in-ter-gă-lak -tik) The matter contained in the space between the galaxies, about which very little is known. It exists in large amounts only in clusters of galaxies where it is completely ionized and constitutes a significant proportion of the total observed mass of the cluster. Absorption lines in quasar spectra often indicate many clouds of intergalactic hydrogen along the line of sight but their total mass is very small.

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.


Vorontsov-Vel’iaminov, B. A. Vnegalakticheskaia astronomiia. Moscow, 1972.
Zel’dovich, la. B., and I. D. Novikov. Reliativistskaia astrofizika. Moscow, 1968.


References in periodicals archive ?
Elvis, "Perspective: missing baryons and the warm-hot intergalactic medium," Science, vol.
If hydrogen atoms in the intergalactic medium had remained unionized, they would have absorbed all the starlight and the universe would have stayed dark.
Neutral hydrogen can be ionized by absorbing ultraviolet light--but reionizing most of the atoms throughout the entire intergalactic medium a billion years after the Big Bang would have required a phenomenal influx of ultraviolet radiation.
Astronomers already had signs that the missing half indeed exists, as a thin, elusive gas between galaxies known as the "warm-hot intergalactic medium," or WHIM.
This latest work supports predictions that it is mostly found in a web of hot, diffuse gas known as the Warm-Hot Intergalactic Medium (WHIM).
The new studies "reveal that most of the visible matter in the universe is in the intergalactic medium," says theorist Jeremiah P.
Finally, HST Ultra-Deep Field observations indicate that early galaxies might have produced sufficient radiation to reionize the intergalactic medium. From cosmic microwave background observations, we know that electrons combined with atoms to form a neutral gas when the universe was about 380,000 years old.
The lack of many bright galaxies at redshift 10 offers clues to what kicked off the Universe's "reionization epoch" - a period between 500 million and 1 billion years after the Big Bang during which luminous objects such as galaxies and quasars ionized the intergalactic medium.
The new data provide the first direct observations that galactic winds interacted with the early intergalactic medium, notes Timothy M.
The metallic atoms are part of the hot gas, or "intergalactic medium," that lies between galaxies.