Radiative Transfer in Ly{\alpha} Nebulae: I. Modeling a Continuous or Clumpy Spherical Halo with a Central Source

• Published in: arXiv.org 2022-12
• Main Authors: Seok-Jun, Chang, Yang, Yujin, Kwang-Il Seon, Zabludoff, Ann, Lee, Hee-Won
• Format: Article
• Language: English
• Subjects: Active galactic nuclei; Alpha rays; Galaxies; Line shape; Line spectra; Nebulae; Polarization; Radiative transfer; Scattering; Star formation; Surface brightness
• ISSN: 2331-8422
• DOI: 10.48550/arxiv.2212.09630

To understand the mechanism behind high-\(z\) Ly\({\alpha}\) nebulae, we simulate the scattering of Ly\({\alpha}\) in a \(\rm H\,I\) halo about a central Ly\({\alpha}\) source. For the first time, we consider both smooth and clumpy distributions of halo gas, as well as a range of outflow speeds, total \(\rm H\,I\) column densities, \(\rm H\,I\) spatial concentrations, and central source galaxies (e.g., with Ly\({\alpha}\) line widths corresponding to those typical of AGN or star-forming galaxies). We compute the spatial-frequency diffusion and the polarization of the Ly\({\alpha}\) photons scattered by atomic hydrogen. Our scattering-only model reproduces the typical size of Ly\({\alpha}\) nebulae (\(\sim 100\,\)kpc) at total column densities \(N_{\rm HI} \geq 10^{20} \rm cm^{-2}\) and predicts a range of positive, flat, and negative polarization radial gradients. We also find two general classes of Ly\({\alpha}\) nebula morphologies: with and without bright cores. Cores are seen when \(N_{\rm HI}\) 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, \(N_{\rm HI}\) dominates the trends. The radial behaviors of the Ly\({\alpha}\) surface brightness, spectral line shape, and polarization in the clumpy model with covering factor \(f_c \gtrsim 5\) approach those of the smooth model at the same \(N_{\rm HI}\). A clumpy medium with high \(N_{\rm HI}\) and low \(f_c \lesssim 2\) generates Ly\({\alpha}\) features via scattering that the smooth model cannot: a bright core, symmetric line profile, and polarization jump.