Work

Lyman-α escape through anisotropic media

Year: 2026

Type: article

Abstract: ABSTRACT The escape of Lyman-alpha (Ly $\alpha$) radiation encodes valuable information on the neutral interstellar medium and is often used as a proxy ionizing photon escape. Yet, the theory of Ly $\alpha$ transfer through anisotropic gas distributions remains underdeveloped. We present Monte Carlo radiative transfer simulations of Ly $\alpha$ propagation through porous, inhomogeneous neutral gas, systematically exploring the effects of channel geometry, outflows, dust, and lognormally distributed column densities. We find that Ly $\alpha$ photons do not preferentially escape through the lowest-column-density pathways, but instead traverse gas of substantial optical depth, leading to suppressed central flux and the absence of strongly beamed escape. Subdividing channels has little impact, indicating geometry and covering fraction are more important than porosity. Channels containing moderate amounts of neutral hydrogen alter escape in characteristic ways, including the appearance of quadruple-peaked spectra, which can be captured by a simple flux–channel relation. Outflows reshape the spectra by facilitating escape through dense media, redshifting photons, and blending central features, while dust modulates the visibility of small channels by suppressing flux at line centre; in both cases, we develop an analytical model that predicts the resulting central fluxes. Extending to lognormal column density fields, we show Ly $\alpha$ photons probe a broad range of optical depths, producing skewed spectra that can be approximated by weighted sums of homogeneous models. Our results have direct implications for using Ly $\alpha$ as a tracer of gas properties and ionizing photon escape; for instance, spectra suggestive of high column densities may nonetheless allow Lyman continuum leakage through narrow channels.

Source: Monthly Notices of the Royal Astronomical Society

DOI: 10.1093/mnras/stag330

Authors: Silvia Almada Monter, Max Gronke, Seok-Jun Chang

Institutions: Max Planck Institute for Astrophysics, Ludwig-Maximilians-Universität München, Heidelberg University

Language: English

Topic: Galaxies: Formation, Evolution, Phenomena

Subfield: Astronomy and Astrophysics

Field: Physics and Astronomy

Domain: Physical Sciences

Open Access status: gold

Estimated APC paid: $3,046

Funders: European Commission, Maritime and Port Authority of Singapore, Max-Planck-Gesellschaft