Interstellar radiation as a Maxwell field: Improved numerical scheme and application to the spectral energy density

Abstract

Abstract The existing models of the interstellar radiation field (ISRF) do not produce a Maxwell field. Here, the recent model of the ISRF as a Maxwell field is improved by considering separately the different frequencies at the stage of the fitting. Using this improved procedure: (i) It is checked in detail that the model does predict extremely high values of the spectral energy density (SED) on the axis of a galaxy, which however decreases very rapidly when ρ \rho , the distance to the axis, is increased from zero. (ii) The difference between the SED values (with ρ = 1 \rho =1 or 8 kpc 8\hspace{0.33em}{\rm{kpc}} ), as predicted either by this model or by a recent radiation transfer model, is reduced significantly. (iii) The slower decrease of the SED with increasing altitude z z , as compared with the radiation transfer model, is confirmed. We also calculate the evolutions of the SED at large ρ \rho . We interpret these evolutions by determining asymptotic expansions of the SED at large z z , and also ones at large ρ \rho .