2023ApJ...945..100C

To understand the mechanism behind high-z Lyα nebulae, we simulate the scattering of Lyα in a H I halo about a central Lyα source. For the first time, we consider both smooth and clumpy distributions of halo gas, as well as a range of outflow speeds, total H I column densities, H I spatial concentrations, and central source galaxies (e.g., with Lyα line widths corresponding to those typical of active galactic nucleus or star-forming galaxies). We compute the spatial-frequency diffusion and the polarization of the Lyα photons scattered by atomic hydrogen. Our scattering-only model reproduces the typical size of Lyα nebulae (∼100 kpc) at total column densities N_H I_ ≥ 10²⁰ cm^–2 and predicts a range of positive, flat, and negative polarization radial gradients. We also find two general classes of Lyα nebula morphologies: with and without bright cores. Cores are seen when N_H I_ is low, i.e., when the central source is directly visible, and are associated with a polarization jump, a steep increase in the polarization radial profile just outside the halo center. Of all the parameters tested in our smooth or clumpy medium model, N_H I_ dominates the trends. The radial behaviors of the Lyα surface brightness, spectral line shape, and polarization in the clumpy model with covering factor f_c ≳ 5 approach those of the smooth model at the same N_H I_. A clumpy medium with high N_H I_ and low f_c ≲ 2 generates Lyα features via scattering that the smooth model cannot: a bright core, symmetric line profile, and polarization jump.