A MODEL FOR THE SCATTERED LIGHT CONTRIBUTION AND POLARIZATION OF THE DIFFUSE Ha GALACTIC BACKGROUND

We present Monte Carlo simulations of the di†use Ha Galactic background. Our models comprise direct and multiply scattered Ha radiation from the kpc scale height warm ionized medium and midplane H II regions. The scattering is o† 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 Ha images that enable us to separate out the contributions of direct and scattered radiation. We also determine how far the model Ha photons have traveled, i.e., how far we see into the Galaxy at Ha. Our models reproduce the overall characteristics of the observed Ha background and predict that the scattered Ha intensity at high latitudes is in the range 5%È20% of the total intensity, in agreement with estimates based on [S II]/Ha and [O III]/Ha line ratio measurements. The polarization arising from dust scattering of Ha from midplane H II regions is predicted to be less than 1% at high latitude. Subject headings : di†use radiation È dust, extinction È H II regions È polarization È radiative transfer