FIR Fine Structure Lines
The far-infrared (FIR) fine-structure lines from carbon, nitrogen, oxygen, and their ions serve as important, and in some cases, dominant coolants of major phases of the interstellar medium. They arise in gas that has been ionized and/or heated by the ultraviolet photons of young stars or active galactic nuclei (AGN). Because the lines are collisionally excited, extinction free and usually optically thin, they cool the gas efficiently, allowing it to collapse and form stars. In many cases one of these lines may be the single brightest emission line from a galaxy with up to one percent of a galaxies’ total luminosity being emitted by one emission line. The far-IR lines are excellent probes for the density and the temperature of the gas from which they were emitted as well as the strength of the exciting UV field; for this reason the detection of even a single line can constrain important internal properties of a galaxy.
Critical density versus ionization potential for several infrared fine-structure emission lines that trace the properties of photo-dissociation regions (green), stellar-HII regions (red), AGN (blue), and coronal emission(black) from Spingolio et al. 2012.