Lyman-alpha emission properties of simulated galaxies: interstellar medium structure and inclination effects

TL;DR

This study investigates how the small-scale structure of the interstellar medium and galaxy orientation influence Lyman-alpha emission properties in simulated galaxies, revealing significant effects on observability and line profiles.

Contribution

It demonstrates the critical role of ISM small-scale structure and inclination in shaping Lyman-alpha signatures, highlighting the need for high-resolution simulations.

Findings

Small-scale ISM structure determines Lya escape fraction and line symmetry.

Galaxy inclination significantly alters Lya line profiles and luminosity.

Inclination effects can bias high-redshift galaxy surveys.

Abstract

[abridged] Aims. The aim of this paper is to assess the impact of the interstellar medium (ISM) physics on Lyman-alpha (Lya) radiation transfer and to quantify how galaxy orientation with respect to the line of sight alters observational signatures. Methods. We compare the results of Lya radiation transfer calculations through the ISM of a couple of idealized galaxy simulations with different ISM models. Results. First, the small-scale structuration of the ISM plays a determinant role in shaping a galaxys Lya properties.The artificially warm, and hence smooth, ISM of G1 yields an escape fraction of 50 percent at the Lya line center, and produces symmetrical double-peak profiles. On the contrary, in G2, most young stars are embedded in thick star-forming clouds, and the result is a 10 times lower escape fraction. G2 also displays a stronger outflowing velocity field, which favors the escape of red-shifted photons, resulting in an asymmetric Lya line. Second, the Lya properties of G2 strongly depend on the inclination at which it is observed: From edge-on to face-on, the line goes from a double-peak profile with an equivalent width of -5 Angstrom to a 15 times more luminous red-shifted asymmetric line with EW 90 Angstrom. Conclusions. Lya radiation transfer calculations can only lead to realistic properties in simulations where galaxies are resolved into giant molecular clouds, putting these calculations out of reach of current large scale cosmological simulations. Finally, we find inclination effects to be much stronger for Lya photons than for continuum radiation. This could potentially introduce severe biases in the selection function of narrow-band Lya emitter surveys, which could indeed miss a significant fraction of the high-z galaxy population.