The Far-UV Interstellar Radiation Field in Galactic Disks: Numerical and Analytic Models

Abstract

Abstract The intensity of the far-ultraviolet (FUV; 6–13.6 eV) interstellar radiation field (ISRF) in galaxies determines the thermal and chemical evolution of the neutral interstellar gas and is key for interpreting extragalactic observations and for theories of star formation. We run a series of galactic disk models and derive the FUV ISRF intensity as a function of the dust-to-gas ratio, star formation rate density, gas density, scale radius, and observer position. We develop an analytic formula for the median FUV ISRF flux. We identify two-dimensionless parameters in the problem: (1) the dimensionless galactic radius, X, which measures the radial extent of FUV sources (OB stellar associations) in the disk; and (2) the opacity over the intersource distance, τ ⋆, which measures the importance of dust absorption. These parameters encapsulate the dependence on all of the physical parameters. We find that there exists a critical τ ⋆, or equivalently a critical dust-to-gas ratio, , the Milky Way value, at which the ISRF changes behavior. For the ISRF is limited by dust absorption. With decreasing , the ISRF intensity increases as more sources contribute to the flux. For the ISRF saturates as the disk becomes optically thin. We find that the ISRF per star formation rate density in low-metallicity systems, such as dwarf and high-redshift galaxies, is higher by up to a factor of 3–6 compared to their Milky Way counterparts. We discuss the implications these findings have for the potential mechanisms that regulate star formation in low-metallicity galaxies.