A public grid of radiative transfer simulations for Lyman-alpha and metal lines in idealised galactic outflows

TL;DR

This paper presents a comprehensive public grid of about 20,000 radiative transfer simulations of Lyman-alpha and metal lines in idealised galactic outflows, aiding interpretation of galaxy gas properties.

Contribution

It introduces a large, accessible simulation grid for Lyman-alpha and metal line spectra in galactic outflows, covering diverse physical conditions and aiding observational analysis.

Findings

Lyman-alpha traces gas at T=10^4-10^5 K even in highly-ionised media.

CII probes similar temperature ranges as Lyman-alpha.

Metal lines' opacity depends on temperature, radiation, metallicity, and dust, affecting their interpretation.

Abstract

The vast majority of star-forming galaxies are surrounded by large reservoirs of gas ejected from the interstellar medium. Ultraviolet absorption and emission lines represent powerful diagnostics to constrain the cool phase of these outflows, through resonant transitions of hydrogen and metal ions. The interpretation of these observations is often remarkably difficult as it requires detailed modelling of the propagation of the continuum and emission lines in the gas. To this aim, we present a large public grid of about 20000 simulated spectra which includes HI Lyman-alpha (Lya) and five metal transitions associated with MgII, CII, SiII, and FeII that is accessible online at https://rascas.univ-lyon1.fr/app/idealised_models_grid/. The spectra have been computed with the RASCAS radiative transfer code for 5760 idealised spherical configurations surrounding a central point source emission, and characterised by their column density, Doppler parameter, dust opacity, wind velocity, as well as various density/velocity gradients. Designed to interpret Lya and metal line profiles, our grid exhibits a wide diversity of resonant absorption and emission features, as well as fluorescent lines. We illustrate how it can help better constrain wind properties by performing a joint modelling of observed Lya, CII, and SiII spectra. Using CLOUDY simulations and virial scaling relations, we show that Lya is expected to be a faithful tracer of the gas at T=10^4-10^5 K, even if the medium is highly-ionised. While CII is found to probe the same range of temperatures as Lya, other metal lines merely trace cooler phases (T=10^4 K). As their gas opacity strongly depends on gas temperature, incident radiation field, metallicity and dust depletion, we caution that optically thin metal lines do not necessarily originate from low HI column densities and may not accurately probe Lyman continuum leakage.