Quantify Lyman-alpha trapping with detailed resonant-transfer microphysics

Quantitatively establish whether detailed resonant-scattering and frequency-redistribution physics for Lyman-alpha maintains a sufficiently large hydrogen $n=2$ population to make the wind opaque to Balmer-continuum photons.

Background

The models use the Sobolev approximation for bound-bound transfer, but Lyman-alpha has broad damping wings for which a full resonant-scattering treatment with frequency redistribution is required. Because Lyman-alpha trapping sustains the hydrogen n=2n=2 population, the treatment can affect the Balmer-continuum opacity and therefore the predicted Balmer cocoon. The paper expects strong trapping qualitatively, but leaves its quantitative verification to more complete microphysics.

References

In our work, the $n=2$ population is sustained by Lyman~$\alpha$ trapping, so Balmer photoionization may be sensitive to these details. Still, given that in either the Sobolev approximation or an improved resonant scattering treatment Lyman~$\alpha$ is strongly trapped, we expect that the $n=2$ population will remain large enough to be opaque to the Balmer continuum. Quantitatively establishing this with more detailed microphysics would be a valuable direction for future work.

— Little Red Dots As Super-Eddington Fountain Flows  (2609.20920 - Kaaz et al., 17 Sep 2026) in Section 5.5, “Caveats and Future Work”