The LWS range includes seven diagnostic fine-structure lines: [CII] 158 µm, [OI] 63 and 146 µm, [NII] 122 µm, [OIII] 52 and 88 µm, and [NIII] 57 µm. The [NIII] 57 µm /[NII] 122 µm line intensity ratio is a sensitive indicator of the hardness of the UV radiation field when T(eff) ~ 33,000 K. This pair thus indicates the upper mass limit (or, age) of any purported starburst. The [OIII] 52,88 µm lines can be used to derive the density for moderate densities of the ionized gas and together with the [NIII] 57 µm and [NII] 122 µm lines can be used to estimate the average O/N abundance ratio, and hence, star formation history in the galaxy. The [NII] 122 µm line can also be used to discern the fraction of [CII] emission which arises from low density ionized gas.
By definition, only photons less energetic than those capable of ionizing hydrogen (i.e. with energies less than 13.6 eV) pass the ionization boundary of an HII region and heat the gas within PDRs. The most energetic of these, the far ultraviolet photons with energies of ~ 6 - 13.6 eV are absorbed by dust followed by the ejection of energetic photoelectrons which then heat the gas in the ensuing layers of H I, [CII] (ionization energy 11.3 eV), [OI] and molecules. Models of the heating and cooling of the gas and dust, including polycyclic aromatic hydrocarbon (PAH) heating, have produced diagnostic diagrams relating the strong cooling line emission and the total far-infrared dust emission to the average radiation density and hydrogen density in the PDR (Wolfire et al. 1990; Kaufman et al. 1999). These models can be applied directly if both the total FIR and the line emission come from the PDR and if the line emission is not affected by absorption due to foreground gas and dust. Since the [CII]158 µm and [OI]63,145 µm, lines from these regions are very strong, they can be used to trace the conditions in dusty star forming regions, even in elliptical and blue compact dwarfs where the amount of dust is relatively low.