Radiation as a physical probe

Our sole window on the environment of molecular clouds is provided by radiation from astrophysical processes. Gas in star-forming environments may both emit and absorb spectral line and continuum radiation, whose properties can be used to derive physical and chemical conditions. This document begins with a description of how radiation and molecules interact to form rotational spectra in the millimetre regime. The statistical emission and absorption properties of the gas are then used to explain how radiation is transmitted through the interstellar medium, allowing the derivation of physical parameters from observations. Lastly, the spectrum of the versatile molecular thermometer NH3 is described and the analysis methodology presented as a case study. The following discussion borrows heavily from Townes & Schawlow (1955), Rohlfs & Wilson (2004) and Goldsmith & Langer (1999), whose formalisms have been adopted here. Unless specifically stated all equations and quantities are presented in the SI (MKS) unit system.