Dust Eddington Ratios for Star-Forming Galaxy Subregions

Abstract: Radiation pressure on dust is an important feedback process around star clusters and may eject gas from bright sub-regions in star-forming galaxies. The Eddington ratio has previously been constructed for galaxy-averaged observations, individual star clusters, and Galactic HII regions. Here we assess the role of radiation pressure in thousands of sub-regions across two local star-forming galaxies, NGC 6946 and NGC 5194. Using a model for the spectral energy distribution from stellar population synthesis and realistic dust grain scattering and absorption, we compute flux- and radiation pressure-mean opacities and population-averaged optical depth $\langle\tau_{\rm RP}\rangle$. Using Monte-Carlo calculations, we assess the momentum coupling through a dusty column to the stellar continuum. Optically-thin regions around young stellar populations are $30-50$ times super-Eddington. We calculate the Eddington ratio for the sub-regions including the local mass of young and old stars and HI and molecular gas. We compute the fraction of the total star formation that is currently super-Eddington, and provide an assessment of the role of radiation pressure in the dusty gas dynamics. Depending on the assumed height of the dusty gas and the age of the stellar population, we find that $\sim0-10$% of the sightlines are super-Eddington. These regions may be accelerated to $\sim5-15$ km/s by radiation pressure alone. Additionally, our results show that for beamed radiation the function $1-\exp(-\langle\tau_{\rm RP}\rangle)$ is an excellent approximation to the momentum transfer. Opacities and optical depths are tabulated for SEDs of different stellar ages and for continuous star formation.