Lyα with SPICE: Interpreting Lyα emission at z>5

Lyα emission is key to understanding the process of cosmic reionisation. JWST is finally enabling us to measure Lyα emission deep into the epoch of reionisation for an increasing number of galaxies. However, discrepancies between measurements of Lyα equivalent widths (EW_0) of Lyα emitters (LAEs) have been noted between JWST and ground-based facilities. We employ SPICE, a suite of radiation-hydrodynamical simulations featuring different stellar feedback models, and investigate the impact of radiative transfer effects and observational systematics on the measured Lyα EW_0. We perform radiative transfer of Lyα and UV photons for SPICE galaxies to mimic slit spectroscopy for ground-based slits and JWST-MSA pseudo-slits. Spatial Lyα-UV offsets exist independently of feedback model, and are common (>70 ≈0.07-0.11” and these predictions are consistent with the observed spatial offsets. Spatial Lyα-UV offsets are a major cause of loss of flux for JWST-MSA observations, with median pseudo-slit losses of ≈65 and ≈30 absence of such spatial offsets, the presence of extended emission can cause median pseudo-slit losses of 40 pseudo-slit losses. Complex galaxy morphologies or misplaced JWST-MSA pseudo-slit can lead to under-estimated UV continuum, resulting in spuriously high estimates of EW_0 from JWST in 6-8 predictions from SPICE to 25 galaxies with Lyα emission observations from both the ground and JWST. The EW_0^ JWST and the EW_0^ Ground exhibit scatter in line with predictions, indicating that both physical and systematic effects are likely at play.