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

Seok-Jun Chang, Yujin Yang+3 more|arXiv (Cornell University)|Dec 19, 2022

Astronomy and Astrophysical Research4 citations

TL;DR

This study models Lyα radiative transfer in spherical H I halos with a central source, comparing smooth and clumpy gas distributions to explain the morphology, polarization, and spectral features of high-redshift Lyα nebulae. It finds that high H I column density (≥10²⁰ cm⁻²) and low covering factor in clumpy media reproduce extended nebulae (~100 kpc), symmetric line profiles, and unique polarization jumps absent in smooth models.

ABSTRACT

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

Motivation & Objective

• To understand the physical mechanisms behind the extended Lyα emission in high-redshift nebulae, particularly Lyα blobs (LABs) and enormous Lyα nebulae (ELANe).
• To investigate how scattering by neutral hydrogen in spherical halos produces observable features such as surface brightness profiles, spectral line shapes, and polarization patterns.
• To determine whether scattering alone can reproduce the observed sizes (~100 kpc) and morphologies of Lyα nebulae, especially in the context of clumpy versus smooth gas distributions.
• To explore the role of source properties—such as Lyα line width (AGN vs. star-forming galaxies), outflow speed, and H I concentration—in shaping polarization and spectral features.
• To provide a theoretical framework for interpreting polarimetric observations of Lyα nebulae, which are increasingly used to distinguish scattering from other emission mechanisms.

Proposed method

• Developed a 3D radiative transfer model for Lyα photons scattering in spherical H I halos with a central source, using Monte Carlo simulations to track photon trajectories.
• Incorporated both smooth and clumpy gas distributions, with the clumpy model parameterized by covering factor (fc) and H I column density (N_HI).
• Tracked polarization evolution by computing the degree of linear polarization (DoP) and its radial profile, accounting for resonant scattering and anisotropic emission.
• Computed spatial-frequency diffusion and spectral line profiles to compare with observations, using Lyα luminosity fixed at 10⁴⁴ erg s⁻¹ at z=3.
• Varied source line widths (σ_src) and outflow speeds (v_exp) to simulate AGN and star-forming galaxy sources, and tested their impact on polarization gradients.
• Validated results by comparing clumpy and smooth models at identical N_HI, assessing convergence and distinguishing features such as polarization jumps and symmetric line profiles.

Experimental results

Research questions

• RQ1 Can scattering alone produce Lyα nebulae with observed sizes of ~100 kpc, and what H I column density is required?
• RQ2 How do clumpy versus smooth gas distributions affect the radial polarization profile, and what features (e.g., polarization jumps) emerge in each case?
• RQ3 What physical conditions (e.g., N_HI, covering factor, source line width) lead to symmetric Lyα line profiles, which are observed in some LABs but not explained by smooth models?
• RQ4 Why do polarization profiles show diverse gradients (positive, flat, negative), and how do source properties like σ_src and v_exp influence this?
• RQ5 Can the clumpy model reproduce observational features—such as bright cores and polarization jumps—that are absent in smooth models?

Key findings

• A total H I column density of N_HI ≥ 10²⁰ cm⁻² is required to produce Lyα halos extending beyond ~50 kpc, matching observed LAB and ELANe sizes.
• Halo morphology splits into two classes: those with a bright core (present at low N_HI) and those without (dominant at high N_HI), with cores linked to a polarization jump just outside the center.
• The radial polarization profile can exhibit positive, flat, or negative gradients depending on the relative values of source line width (σ_src) and outflow speed (v_exp), especially when σ_src ≈ v_exp.
• In clumpy models with low covering factor (fc ≤ 2) and high N_HI, the Lyα line profile becomes symmetric and peaks at systemic velocity—features not reproduced by smooth models.
• The polarization jump near the halo center, associated with single-wing scattering, disappears at high N_HI (≥10²¹ cm⁻²), where multiple wing scatterings dominate.
• For fc ≳ 5, the clumpy model's surface brightness, spectral, and polarization profiles converge with those of the smooth model at the same N_HI, indicating a transition to smooth-like behavior.