An Etymological Dictionary of Astronomy and Astrophysics
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The interface between a galaxy and the → intergalactic medium. The circumgalactic medium comprises gas located in the → halo of a galaxy extending out to the → virial radius.

→ circum-; → galactic.

The interstellar matter outside condensed molecular clouds. Diffuse interstellar medium consists of a hot intercloud medium, a warm intercloud medium, and a cold neutral medium with hydrogen atom densities n_H ~ 0.003, ~ 0.25, and ~ 40 cm^-3, and mean gas → kinetic temperatures T_k ~ 5 x 10⁵, ~ 10⁴, and 80 K, respectively.

→ diffuse; → interstellar; → medium.

A medium in which the → phase velocity is a function of → wave number (→ frequency).

→ dispersive; → medium.

A medium in which several molecular clouds are situated.

→ inter- + → cloud + → medium.

The diffuse medium between → clumps inside → molecular clouds.

→ inter- + → clump + → medium.

The matter lying between the clusters of galaxies in an aggregation of such clusters.

→ inter- + → cluster + → medium.

The matter or environment between the galaxies of a → cluster.

→ intergalactic; → medium.

The material contained in the → solar system in the space through which the planets travel. It consists of the smaller objects such as → asteroids, → comets, → meteorites and also a general pervading → interplanetary dust. Moreover a → plasma of charged particles streaming outward from the Sun in the form of the → solar wind contributes to the interplanetary medium.

→ interplanetary; → medium.

The environment containing the → interstellar matter, consisting of gas (mostly hydrogen) and dust. Even at its densest phase, the interstellar medium is emptier than the best vacuum man can create in the laboratory, but because space is so vast, the interstellar medium still adds up to a huge amount of mass.

→ interstellar; → medium.

A diffuse (N_e ~ 10^-3 cm^-3), hot (T ~ 10⁷-10⁸ K), magnetized (B ~ 0.1-10 μG) plasma that exists between galaxies in a → galaxy cluster and is composed mainly of H, He, and → heavy elements. The ICM strongly emits → X-rays (L_x ~ 10⁴⁵ erg s^-1), making it the most luminous extended X-ray source in Universe. While some of the gas has been stripped out of galaxies, it is also likely that some is also primordial in nature, and has been accreted into the clusters. The origin of the ICM is subject of intense investigation. Broadly, two possibilities have been envisaged. The first one considers the intracluster gas to be once contained in galaxies and later driven in the ICM. This would explain several observations: the presence of high → metallicity gas, and H I deficiency of galaxies residing in the cores of rich clusters (which suggests that gas stripping has occurred). Alternatively, the ICM is primordial, originating at the time of cluster formation. Actually the ICM may result from a combination of both scenarios.

→ intra-; → cluster; → medium.

An intervening substance through which an effect is produced; surrounding objects, conditions, or influences; environment. → interstellar medium.

From L. medium, from neut. of adj. medius "middle," from PIE *medhyo- "middle;" cf. Av. maδəma- "middle," as below.

Madim, from Av. maδəma- [adj.] "middle, being in the middle; middling, of a middling size or quality," maiδim "in the midst of," maiδyāna- "the middle;" cf. Skt. mádhya- "middle;" Gk. messos; Goth. midjis; O.E. midd "middle;" O.C.S. medzu "between," Arm. mej "middle;" L. medium, as above.

A medium in which the → phase velocity is independent of frequency.

→ non-; → dispersive; → medium.

A component of the → interstellar medium consisting of an extremely tenuous (density 0.1 to 10 cm^-3) and relatively warm gas (temperature about 8,000 K) filling the space between denser neutral and ionized gas. Hydrogen is partly ionized, partly atomic and observed by the → 21-centimeter line in emission.

→ warm; → intercloud medium.

The space containing a cluster of galaxies filled with a tenuous gas of temperature 10⁵ to 10⁷ K and density 10^-6 to 10^-4 cm^-3. WHIM has been continuously shock-heated during the process of structure formation. It is so highly ionized that it can only absorb or emit far-ultraviolet and soft X-ray photons, primarily at spectral lines of highly ionized C, O, Ne, and Fe. WHIM is thought to be the main reservoir of missing baryons.

→ warm; → hot; → intergalactic medium.