Simulating the diversity of shapes of the Lyman-α line

The Ly α line is a powerful probe of distant galaxies, which contains information about inflowing/outflowing gas through which Ly α photons scatter. To develop our understanding of this probe, we post-process a zoom-in radiation-hydrodynamics simulation of a low-mass (M_star ~ 10⁹ M_⊙) galaxy to construct 22 500 mock spectra in 300 directions from z = 3 to 4. Remarkably, we show that one galaxy can reproduce the variety of a large sample of spectroscopically observed Ly α line profiles. While most mock spectra exhibit double-peak profiles with a dominant red peak, their shapes cover a large parameter space in terms of peak velocities, peak separation, and flux ratio. This diversity originates from radiative transfer effects at interstellar medium and circum-galactic medium (CGM) scales, and depends on galaxy inclination and evolutionary phase. Red-dominated lines preferentially arise in face-on directions during post-starburst outflows and are bright. Conversely, accretion phases usually yield symmetric double peaks in the edge-on direction and are fainter. While resonant scattering effects at <0.2 × R_vir are responsible for the broadening and velocity shift of the red peak, the extended CGM acts as a screen and impacts the observed peak separation. The ability of simulations to reproduce observed Ly α profiles and link their properties with galaxy physical parameters offers new perspectives to use Ly α to constrain the mechanisms that regulate galaxy formation and evolution. Notably, our study implies that deeper Ly α surveys may unveil a new population of blue-dominated lines tracing inflowing gas.