The SPHINX public data release. II. Using low-ionisation absorption lines and dust attenuation to predict Lyman continuum escape

Low-ionisation state (LIS) metal absorption lines, such as siiilineb, are widely used to trace the properties and dynamics of the interstellar medium (ISM) in galaxies. These lines provide crucial insights into galaxy evolution, including feedback mechanisms, metal enrichment, and the escape fraction of ionising photons (fesc) during the epoch of reionisation. We expand our understanding of LIS absorption lines as diagnostic tools for ISM properties and fesc. Using the high-resolution sphinx 20 cosmological radiation-hydrodynamics simulation, we generated a comprehensive synthetic dataset of LIS absorption lines and tested their predictive power for in star-forming galaxies. Synthetic ISM absorption lines, focusing on siiiline and siiilineb, were computed with the radiative transfer code rascas, incorporating resonant scattering of photons, fluorescent emission, and interactions with dust grains. The simulated data enhance the public sphinx 20 dataset with high-resolution LIS lines for the full 1380 galaxies and ten viewing angles per galaxy. We analysed correlations between line properties (width, depth, and Doppler shift), dust attenuation, and extending previous single-galaxy studies to a statistically significant mock galaxy sample. We also tested our predictions on observed data using the LzLCS and CLASSY surveys. We found a strong correlation between the dust-corrected residual flux of siiilineb, tilde R ≡ ⋅ 10^ R_ flux ^ 1526 -0. 4A_ 1500, and fesc. More precisely, we found fesc ≈ 1. 041 ^ R 1. 887 - 0. 002, with an average absolute error of 0. 02. When we applied observational conditions, the error increased, but the escape fraction was still well recovered. In particular, the measurement of residual fluxes required a very high spectral resolution, and the dust attenuation is not directly observable. We show by applying common tools for fitting the spectral energy distribution to our mock data that the inferred dust attenuation is often far from the correct value, with a tendency to underestimate the attenuation when the effect of dust is strongest. Our results demonstrate that the residual flux of siiilineb is a powerful predictor of the escape fraction of ionising photons when it is corrected for dust. The spectra, line measurements, and escape fraction values used in this work are made publicly available.