Abstract

Contributed Talk - Splinter Galaxies

Friday, 11 September 2026, 17:15   (MW-0250)

What Shapes Lyman-alpha Spectra? Radiative Transfer through Anisotropic Gas

Silvia Almada Monter
Max Planck Institute for Astrophysics

In recent years, thanks to the James Webb Telescope (JWST), an increasing number of puzzling observations of the Lyman-alpha (Lya) line have been made from the high-redshift Universe. Since Lya is sensitive to gas density, dust, and fragmentation, it provides crucial information about the structure of the interstellar medium (ISM) and circumgalactic medium (CGM) in high-redshift galaxies. It is particularly relevant for studies of the Epoch of Reionization, because Lya and ionizing photons interact with the same neutral hydrogen. Therefore, the escape of Lya photons from their host galaxies not only reveals the underlying gas distribution but also provides indirect insight into the escape of ionizing radiation. Because ionizing photons are easily absorbed by neutral gas, they can escape galaxies only through low-density channels created by feedback-driven outflows and ISM clearing. To quantify how such porous gas structures shape Lya observables, we performed Monte Carlo radiative-transfer simulations through anisotropic media featuring a range of low-density channel geometries, outflow configurations, dust contents, and column-density distributions (including realistic lognormal profiles). We modelled Lya radiative transfer in both idealized configurations, designed to isolate specific physical effects, and high-resolution multiphase ISM simulations that capture the complexity of realistic galactic environments. We found that Lya photons can escape not only through low-column-density channels ("paths of least resistance") but also through high-column-density regions. In other words, Lya probes both dense and diffuse gas simultaneously and is a richer tracer of the gas distribution than previously thought. We validated these theoretical predictions using over 1000 simulations spanning a wide range of column densities, velocity fields, and geometrical configurations. These findings challenge long-standing expectations and provide new tools for connecting the gas geometry of high-redshift galaxies with their observed Lya spectra.