Radiative Transfer in Lyα Nebulae: I. Modeling a Continuous or Clumpy Spherical Halo with a Central Source

Abstract

To understand the mechanism behind high-$z$ Ly$\alpha$ nebulae, we simulate the scattering of Ly$\alpha$ in a $\mathrm{H}\mathrm{I}$ 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 $\mathrm{H}\mathrm{I}$ column densities, $\mathrm{H}\mathrm{I}$ 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 $N_{\mathrm{HI}}\ge 10^{20}\mathrm{c}{\mathrm{m}}^{-2}$ 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 $N_{\mathrm{HI}}$ 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, $N_{\mathrm{HI}}$ 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 $N_{\mathrm{HI}}$. A clumpy medium with high $N_{\mathrm{HI}}$ 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.

42 pages, 27 figures, accepted for publication in ApJ, Comments welcome!

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• Spatially-resolved Spectroscopic Analysis of Ly$\alpha$ Haloes: Radial Evolution of the Ly$\alpha$ Line Profile out to 60 kpc
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• Revealing the Gas Recycling in the Circumgalactic Medium (CGM) Utilizing a Luminous Ly$\alpha$ Nebula Around a Type-II Quasar at z=2.6 with the Keck Cosmic Web Imager (KCWI)
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• QSO MUSEUM III: the circumgalactic medium in Ly$\alpha$ emission around 120 $z\sim 3$ quasars covering the SDSS parameter space. Witnessing the instantaneous AGN feedback on halo scales
• ODIN: A New Lyman Alpha Blob Selection Method, Sample, and Statistical Analysis at $z\sim 3.1$
• Constraining the geometry of the gas surrounding a typical galaxy at $z=3.4$ with Ly$\alpha$ polarization
• Exploring the gaseous halos of z>3 radio galaxies with UVES and JWST/NIRSpec