The SPHINX public data release

II. Using low-ionisation absorption lines and dust attenuation to predict Lyman continuum escape

¹ Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV, Groningen, The Netherlands
² Centre de Recherche Astrophysique de Lyon UMR5574, Univ Lyon1, ENS de Lyon, CNRS, F-69230 Saint-Genis-Laval, France
³ Observatoire de Genève, Université de Genève, Chemin Pegasi 51, 1290 Versoix, Switzerland
⁴ Aix Marseille Univ., CNRS, CNES, Laboratoire d’Astrophysique de Marseille, F-13388 Marseille, France
⁵ Department of Astronomy, The University of Texas at Austin, 2515 Speedway, Stop C1400 Austin, TX 78712, USA
⁶ Cosmic Frontier Center, The University of Texas at Austin, Austin, TX 78712, USA
⁷ Department of Astronomy, Oskar Klein Centre, Stockholm University, 106 91 Stockholm, Sweden
⁸ LUX, Observatoire de Paris, Université PSL, Sorbonne Université, CNRS, 75014 Paris, France
⁹ Department of Astronomy, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 03722, Republic of Korea
¹⁰ Observatoire Astronomique de Strasbourg, Université de Strasbourg, CNRS UMR 7550, 11 rue de l’Université, 67000 Strasbourg, France
¹¹ Department of Astrophysical Sciences, Princeton University, Peyton Hall, Princeton, NJ 08544, USA

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 28 November 2025
Accepted: 4 March 2026

Abstract

Context. Low-ionisation state (LIS) metal absorption lines, such as Si II λ1526, 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 (f_esc) during the epoch of reionisation.

Aims. We expand our understanding of LIS absorption lines as diagnostic tools for ISM properties and f_esc. Using the high-resolution SPHINX²⁰ cosmological radiation-hydrodynamics simulation, we generated a comprehensive synthetic dataset of LIS absorption lines and tested their predictive power for f_esc in star-forming galaxies.

Methods. Synthetic ISM absorption lines, focusing on Si II λ1260 and Si II λ1526, 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 SPHINX²⁰ 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 f_esc, 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.

Results. We found a strong correlation between the dust-corrected residual flux of Si II λ1526, R∼ ≡ R_flux¹⁵²⁶ · 10^{−0.4A₁₅₀₀}, and f_esc. More precisely, we found f_esc ≈ 1.041R∼^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.

Conclusions. Our results demonstrate that the residual flux of Si II λ1526 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.