Deciphering the Lyman-α emission line: towards the understanding of galactic properties extracted from Lyα spectra via radiative transfer modelling

Abstract

Existing ubiquitously in the Universe with the highest luminosity, the Lyman-α (Lyα) emission line encodes abundant physical information about the gaseous medium it interacts with. Nevertheless, the resonant nature of the Lyα line complicates the radiative transfer (RT) modeling of the line profile. We revisit the problem of deciphering the Lyα emission line with RT modeling. We reveal intrinsic parameter degeneracies in the widely-used shell model in the optically thick regime for both static and outflowing cases, which suggest the limitations of the model. We also explore the connection between the more physically realistic multiphase, clumpy model and the shell model. We find that the parameters of a “very clumpy” slab model and the shell model have the following correspondences: (1) the total column density, the effective temperature, and the average radial clump outflow velocity of the clumpy slab model are equal to the H i column density, effective temperature, and expansion velocity of the shell model, respectively; (2) large intrinsic line widths are required in the shell model to reproduce the wings of the clumpy slab models; (3) adding another phase of hot inter-clump medium increases peak separation, and the fitted shell expansion velocity lies between the outflow velocities of two phases of gas. Our results provide a viable solution to the major discrepancies associated with Lyα fitting reported in previous literature, and emphasize the importance of utilizing information from additional observations to break the intrinsic degeneracies and interpreting the model parameters in a more physically realistic context.

Lyα Scattering Models Trace Accretion and Outflow Kinematics in T Tauri Systems*

“…Previous modeling work found that the velocities of expanding (outflowing) or collapsing (accreting) shells are well-constrained for emission lines with distinct, asymmetric double peaks (Li & Gronke 2022). Figure 6 compares the observed Lyα flux ratios to the H I velocities derived from the shell models in this work that do and do not account for resonant scattering.…”

“…However, it is clear that a real scattering geometry is neither isotropic nor does it have constant velocity and density profiles as assumed in the shell model. There is, therefore, an ongoing debate in the literature regarding the usefulness of shell model fitting as well as the physical interpretation of the derived parameters (e.g., Orlitova et al 2018;Li & Gronke 2022). What is clear from this debate is that shell model parameters cannot be interpreted literally, if at all, but instead represent some weighted quantity probed by Lyα photons.…”

“…The velocities of absorbing H I are the only parameters that are statistically significantly correlated with the observed emission line profile shapes, and values extracted from models when scattering is and is not included are almost always consistent within the 1σ uncertainties. One has to bear in mind that degeneracies between the parameters exist, e.g., between N(H I), the intrinsic emission line width, and the effective temperature of the medium (Li & Gronke 2022). It is likely then that the best-fit scattering models with the smallest H I column densities (N H 16.3, 18.2 I ( ) = for T Cha and CVSO 146, respectively) diverged from the solutions with larger column densities and narrower FWHMs.…”

“…Figure 6 demonstrates that no strong correlations are detected between the Lyα flux ratios and H I column densities (Spearman ρ = −0.1, p = 0.6 when scattering is not included; Spearman ρ = 0.08, p = 0.7 with scattering) or FWHMs of the intrinsic emission lines (Spearman ρ = −0.2, p = 0.5 without scattering; Spearman ρ = −0.3, p = 0.1 when scattering is included). Li & Gronke (2022) find that N(H I), emission line widths, and effective temperatures within the scattering medium can be degenerate parameters, which may be responsible for the lack of correlations with the observed flux ratios (see corner plot for Sz 111 in Appendix A). The intrinsic line widths are challenging to constrain, because of both the geocoronal emission masking velocities <250 km s −1 and turbulence within the accretion shocks themselves.…”

Scrutiny of a very young, metal-poor star-forming Lyα-emitter at z ~ 3.7

“…Orlitová et al 2018), the intrinsic width of Ly𝛼 recovered by the radiative transfer models results to be significantly larger than the one of the Balmer lines (for our target a factor ∼ 3.4 larger if compared to our estimate of the H𝛽 line, in line with the results from Orlitová et al 2018). According to Li & Gronke (2022), however, this discrepancy is most likely due to additional radiative transfer effects and, in particular, the possibility that the Ly𝛼 propagates within a clumpy gas distribution with velocity dispersions 100 km/s. Since 'shell-models' are an oversemplification of the complex structure and kinematics of Ly𝛼-emitters and their surroundings, it is currently debated how much of the radiative transfer is indeed captured by the models and the real meaning and reliability of the involved parameters, especially T and 𝜏 𝑑 .…”