Modeling the spectral energy distribution of starburst galaxies The role of photodissociation regions

Abstract

Analyzing multiwavelength observations of galaxies from the far-ultraviolet to the millimeter domains provides a wealth of information on the physical properties of galaxies and their evolution across cosmic time. Existing or upcoming ground-based or space-borne facilities with enhanced sensitivities and resolutions open an unprecedented window on the galaxy evolution in the early Universe. However, the derivation of galaxy properties from nebular emission lines is not trivial because the interstellar medium in a galaxy may be patchy, and emission might originate both from starburst emission regions and from partially covered photon-dominated regions. We model both the nebular continuum emission and the line emission of the spectral energy distribution for galaxies exhibiting both a HII region-like emission and emission like that from a photon-dominated regions to account for the partial shielding of the starburst emission region by dense clouds. Nebular galactic emission was modeled from far-ultraviolet to millimeter ranges in a two-sector model with an HII region and a photon-dominated region. The partial overlap of the HII region by the photon dominated region was accounted for by a covering factor. We generated grids of emission spectra using the Cloudy photoionization code for our two-sector model. We compared our models with spectral lines from different samples of galaxies for which we mixed characteristic emission from starburst regions and denser regions. We show that the infrared line ratios can constrain the density, metallicity, photoionization parameter, and the covering factor. We also built infrared diagnostic diagrams based on different infrared line ratios in which the galaxy location contains information about its physical conditions. The two-sector model that couples starburst emission regions and photon-dominated regions can span the existing observations. We implement the resulting emission line libraries in the CIGALE galaxy spectral energy distribution code to help interpret spectrophotometric observations.