Photodissociation and X-Ray Dominated Regions

The radiation from stars and active galactic nuclei (AGN) creates photodissociation regions (PDRs) and X-ray dominated regions (XDRs), where the chemistry or heating are dominated by far-ultraviolet (FUV) radiation or X-ray radiation, respectively. PDRs include a wide range of environments from the diffuse interstellar medium to dense star-forming regions. XDRs are found in the center of galaxies hosting AGN, in protostellar disks, and in the vicinity of X-ray binaries. In this review, we describe the dominant thermal, chemical, and radiation transfer processes in PDRs and XDRs, as well as a brief description of models and their use to analyze observations. We then present recent results from Milky Way, nearby extragalactic, and high-redshift observations. Several important results are: $\bullet$ Velocity resolved PDR lines reveal the kinematics of the neutral atomic gas and provide constraints on the stellar feedback process. Their interpretation is, however, in dispute as observations suggest a prominent role for stellar winds while they are much less important in theoretical models. $\bullet$ A significant fraction of molecular mass resides in CO-dark gas especially in low-metallicity/highly irradiated environments. $\bullet$ The CO ladder and CI/CII ratios can determine if FUV or X-rays dominate the ISM heating of extragalactic sources. $\bullet$ With ALMA, PDR and XDR tracers are now routinely detected on galactic scales over cosmic time. This makes it possible to link the star formation history of the Universe to the evolution of the physical and chemical properties of the gas.