Interstellar Medium

The interstellar medium (ISM) is the matter filling the space between stellar systems in galaxies. ISM includes atomic and molecular gas (99% of mass), dust particles, cosmic rays and the dark matter.

The density of interstellar medium is extremely low. In dense regions of the ISM (molecular clouds) the density reaches 10^6/cm^3, while in diffuse regions of the ISM, where matter is primarily ionized, the density is just some 0.0001 ions per cm^3. The average density of HI interstellar clouds is in the range 0.1-1000 atoms per cubic cm (For comparison, the air we breathe has a density of approximately 10^19 molecules per cubic cm).

The ISM plays a crucial role in the Universe. Stars form within the densest regions of the ISM, while the chemical composition of ISM is a result of primordial nucleosynthesis and enrichment in the process of stellar evolution. Throughout his life, stars emit a stellar wind, enriching the surrounding area with the material from the stellar atmosphere. At the end of their life, stars explode as supernovae or throw our the outer shell in form of planetary nebula, enriching the interstellar medium with energy and products of nuclear fusion.

Spectroscopic investigations of the interstellar medium started from the discovery of so-called “stationary line” in the spectrum of delta Ori by Hartmann in 1904 (ApJ, 19, 268) who found that “the calcium line at 3934 Angstrom does not share in the periodic displacements of the lines caused by the orbital motion of the star”. Since that time many interstellar atomic and molecular lines and the unidentified diffuse interstellar bands (DIB) have been discovered in spectra of reddened, early type stars. A vast majority of the identified interstellar absorption features (atomic and molecular) is concentrated in the blue – near-UV spectral range. Numerous unidentified diffuse interstellar bands have been found mostly in the yellow-red part of the electromagnetic spectrum.

The problem of dentification of carriers of DIB is longest standing problem in all of spectroscopy. First diffuse bands were discovered in 1921 by L.Heger at Lick Observatory. Currently, the list of known DIBs contains more than 400 entries; a majority of them – very shallow. The fine structure (very likely due to the rotational structure of polyatomic molecules) has been detected in some DIBs.  Nearly all conceivable forms of matter – from hydrogen anion to dust grains – have already been proposed as DIB carriers, so far with no generally accepted success. It should be noted that their variable strength ratios demonstrate a variety of carriers, and thus strongly support their molecular origin. Various candidates have been proposed as possible carriers for the bands, ranging from dust grains, to free, neutral and ionized, molecular species of varying sizes and structures.

It is now clear that the DIBs cannot be explained by the early concept of a single carrier due to the large number of bands detected and the lack of correlation between their strengths. It is commonly believed now that DIBs originate in complex molecules, most probably carbon-bearing ones. The latter are believed to be either in the form of polycyclic aromatic hydrocarbons (PAHs), fullerenes and/or linear carbon molecules. DIBs are ubiquitous – they are observed in all diffuse clouds of our Galaxy and in other galaxies. Rough estimations lead to conclusion that but our Galaxy contains some 10^58 unidentified large organic molecules!

Eagle Nebulae

Faculty member active in this area: Gazinur Galazutdinov