A Model for the Scattered Light Contribution and Polarization of the Diffuse H$α$ Galactic Background

Abstract

We present Monte Carlo simulations of the Diffuse H$α$ Galactic Background. Our models comprise direct and multiply scattered H$α$ radiation from the kpc scaleheight Warm Ionized Medium and midplane H II regions. The scattering is off dust that is assumed to be well mixed with the gas, with an axisymmetric density distribution taken from the literature. The results of our simulations are all-sky H$α$ images that enable us to separate out the contributions of direct and scattered radiation. We also determine how far the model H$α$ photons have traveled, i.e., how far we see into the Galaxy at H$α$. Our models reproduce the overall characteristics of the observed H$α$ background and predict the scattered H$α$ intensity at high latitudes is in the range 5% to 20% of the total intensity, in agreement with estimations based on [S II]/H$α$ and [O III]/H$α$ line ratio measurements. The polarization arising from dust scattering of H$α$ from midplane H II regions is predicted to be less than 1% at high latitude.