Interstellar medium infrared emission

The far infrared and submillimetre ranges cover the brighter emission lines of the interstellar medium. This should allow detailed studies of its composition, both in the Milky Way and in other galaxies. 

The earliest detailed studies of silicate dust in protoplanetary disks targeted those brightest in the mid-infrared, where high quality spectra could be obtained even by severely flux-limited observations. Cohen Wittebom (1985) reported the earliest detection of crystalline silicate emission from the environment of young stars and interpreted it as evidence for dust having been transformed from its pristine state in the interstellar medium to the material known to be contained in the comets and perhaps primitive meteorites. Interestingly, this observation and explanation pre-dated the evidence that young stars are surrounded by disks and not by spherical envelopes. 

Whereas on Earth, PAHs and related species such as (benzo[a]pyrene) are considered as highly carcinogenic, mutagenic, and teratogenic, and therefore resemble unwanted by-product in combustion processes, the situation in quite reversed in hydrocarbon-rich atmospheres of planets and their moons, as well as in the interstellar medium. Here, PAH-like species are thought to contribute to the unidentified infrared emission bands (UIBs) observed between 3 and 15 )o,m. " Itis also estimated that PAHs and related molecules such as their radicals, ionized PAHs, and heteroaromatic PAH make up to 20% of the total cosmic carbon budget. Also, the role of PAHs in astrobiology should be noted. ... 

Infrared emission from many regions of the interstellar medium is dominated by what are often called the unidentified infrared bands (UIBs).As shown in Fig. 1.3, these broad features are observed at several wavelengths between 3 and 15 /rm [18]. They are attributed to vibrational transitions associated with the C—H and C—C stretching vibrations (at ca. 3.3 and 6-8 pm, respectively) and CH in-plane and CH out-of-plane bending modes (at ca. 8.5 and 10-15 pm, respectively) in large... 

In the far infrared, the continuum radiation from the interstellar medium is dominated by the dust thermal emission. At long wavelengths in the far infrared and sub-millimetre domains, the emission is dominated by the sub-micrometre dust grains that reach a stable temperature. The resulting emission can be described by a modified black-body law, namely ... 

Hundreds of pure rotational lines of a variety of molecules have been detected in the interstellar medium, most of them in emission but some also in absorption. These lines occur throughout the millimetre, centimetre and decametre wavelength region of the spectrum and are detected using radio antennas and heterodyne detection techniques at spectral resolutions of typically 300 000. At submillimetre and far-infrared wavelengths, the atmosphere is not very transparent (Figure 1) and telescopes are located on high, dry sites, on airborne platforms or in... 

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