A Model for the Scattered Light Contribution and Polarization of the Diffuse H-alpha Galactic Background

We present Monte Carlo simulations of the diffuse H alpha Galactic background. Our models comprise direct and multiply scattered H alpha radiation from the kpc scale height 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 alpha images that enable us to separate out the contributions of direct and scattered radiation. We also determine how far the model H alpha photons have traveled, i.e., how far we see into the Galaxy at H alpha. Our models reproduce the overall characteristics of the observed H alpha background and predict that the scattered Ha intensity at high latitudes is in the range 5%-20% of the total intensity, in agreement with estimates based on [S II]/H alpha and [O III]/H alpha line ratio measurements. The polarization arising from dust scattering of H alpha from midplane H II regions is predicted to be less than 1% at high latitude.