Lyman-alpha resonant-line radiative transfer in expanding media

Abstract

The Lyman-alpha (LyA) line of atomic hydrogen encodes crucial information about both the intrinsic sources and surrounding environments of star-forming regions throughout the Universe. Due to the complexity of resonant scattering, analytic solutions remain scarce, with most studies focusing on idealized, static configurations. However, observations of LyA emitting galaxies consistently reveal signatures of outflows, imprinted through red-peak dominance in spectral line profiles. In this paper, we derive novel analytic solutions for resonant-line radiative transfer in moving media, specifically homologous-like cloud expansion and unbounded cosmological flows, which capture the main physics of velocity gradients. To validate these analytic solutions and identify regimes where diffusion-based assumptions hold, we introduce a robust Gridless Monte Carlo Radiative Transfer (GMCRT) method. By integrating optical depths exactly in the comoving frame, GMCRT updates photon frequencies continuously to account for Doppler shifts induced by velocity gradients. We demonstrate excellent consistency between GMCRT and our analytic solutions in regimes where diffusion approximations apply. At higher velocities or lower optical depths, discrepancies highlight the limitations of simplified formalisms. We also provide scaling relations for a point source in a cloud with a maximum-to-thermal velocity ratio beta = V_max / v_th, as modifying the standard dependence on line centre optical depth of (atau_0)^1/3 by additional factors, e.g. characteristic escape frequency scale as x_esc ~ beta^1/3, force multipliers as M_F ~ beta^-1/3, and trapping time as t_trap ~ beta^-2/3. Our work complements numerical simulations by improving physical intuition about nonstatic environments when interpreting LyA observations and guiding future subgrid prescriptions in galaxy formation models.

20 pages, 18 figures. MNRAS, 541, 179

References in corpus (20)

• 3D Lyman-alpha radiation transfer. I. Understanding Lyman-alpha line profile morphologies
• Lyman Alpha Emitting Galaxies as a Probe of Reionization
• Understanding the escape of LyC and Ly$\alpha$ photons from turbulent clouds
• The THESAN project: Lyman-alpha emission and transmission during the Epoch of Reionization
• Lyman alpha Radiative Transfer in Cosmological Simulations using Adaptive Mesh Refinement
• Lyman-alpha radiative transfer during the Epoch of Reionization: contribution to 21-cm signal fluctuations
• The Lyman-alpha signature of the first galaxies
• Impact of Lyman alpha pressure on metal-poor dwarf galaxies
• The physics of Lyman-alpha escape from disc-like galaxies
• Simulating the diversity of shapes of the Lyman-$\alpha$ line
• Lyman-alpha radiation hydrodynamics of galactic winds before cosmic reionization
• A systematic study of the escape of LyC and Ly$\alpha$ photons from star-forming, magnetized turbulent clouds
• Ly-alpha Radiative Transfer: Monte-Carlo Simulation of the Wouthuysen-Field Effect
• Lyman-alpha radiative transfer: Modeling spectrum and surface brightness profile of Lyman-alpha emitting galaxies at z=3-6
• Lyman-$\alpha$ feedback prevails at Cosmic Dawn: Implications for the first galaxies, stars, and star clusters
• On the effect of Lyman alpha trapping during the initial collapse of massive black hole seeds
• Lyman-alpha radiation pressure: an analytical exploration
• Resonant-line radiative transfer within power-law density profiles
• The impact of gas bulk rotation on the lyman-alpha line
• A public grid of radiative transfer simulations for Lyman-alpha and metal lines in idealised galactic outflows

Cited by in corpus (1)

• Lyman-$\alpha$ radiation pressure regulates star formation efficiency