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

Abstract

Low-ionisation state (LIS) absorption lines, such as SiII 1526, are widely used to trace the properties 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 ($f_{\rm{esc}}$). We expand our understanding of LIS absorption lines as diagnostic tools for ISM properties and $f_{\rm{esc}}$. Using the SPHINX20 cosmological radiation-hydrodynamics simulation, we generated a comprehensive synthetic dataset of LIS absorption lines and tested their predictive power for $f_{\rm{esc}}$ in star-forming galaxies. Synthetic SiII 1260 and SiII 1526 lines 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, dust attenuation, and $f_{\rm{esc}}$. 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 SiII 1526, $\tilde{R} \equiv \rm{R_{flux}^{1526}} \cdot 10^{-0.4A_{1500}}$, and $f_{\rm{esc}}$. We found $f_{\rm{esc}} \approx 1.041\tilde{R}^{1.887} - 0.002$, with small error bars. When we applied observational conditions, the error increased, but the escape fraction was still well recovered. 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 an underestimation of the attenuation when the effect of dust is strongest. Our results demonstrate that the residual flux of SiII 1526 is a powerful predictor of the escape fraction of ionising photons.

Figures (12)

• Figure 1: Energy levels of the $\rm{Si}^+$ ion and transitions of $\rm{Si \, \textsc{ii} \, \lambda} 1526$. $P_{31}$ and $P_{32}$ are the probabilities that a $\rm{Si}^+$ ion in level 3 radiatively de-excites to level 1 or 2.
• Figure 2: Examples of $\rm{Si \, \textsc{ii} \, \lambda} 1526$ absorption and fluorescent emission profiles drawn from different haloes and LOS among our seven simulation snapshots. The spectra were selected using KMeans clustering to span a diverse range of spectral properties, including the equivalent width of absorption, residual flux, velocity centroid, and fluorescence strength. Each panel shows the normalised spectrum (blue) along with a dashed horizontal line at the minimum flux level, and a vertical dotted grey line at line centre. The red line highlights the value of the residual flux, which is indicated at the top of each panel, along with the corresponding $f_{\rm{esc}}$.
• Figure 3: Comparison of the escape fraction integrated over all ionising wavelengths on the x-axis and the escape fraction between $890 \hbox{\normalfont\AA}$ and $910 \hbox{\normalfont\AA}$ on the y-axis for all our simulated galaxies in the seven snapshots and in ten viewing angles.
• Figure 4: Relations between the escape fraction of ionising photons and three $\rm{Si \, \textsc{ii} \, \lambda} 1526$ properties. The solid lines show the running median, and the dashed lines show the $16^{\rm{th}}$ and $84^{\rm{th}}$ percentiles. These lines are affected by spectra with $\log(f_{\rm{esc}})<-3$, which are not displayed here. The left and middle panels show the residual flux and equivalent width of the absorption line, respectively. The dotted pink line shows the upper limit found in Mauerhofer21. The right panel shows the centroid velocity of $\rm{Si \, \textsc{ii} \, \lambda} 1526$.
• Figure 5: Comparison of the escape fraction of ionising photons with the product of $\rm{Si \, \textsc{ii} \, \lambda} 1526$ residual flux and the dust attenuation. The colour scale shows the number of spectra in each hexagonal bin. The solid orange line shows the running median, and the dashed lines show the $16^{\rm{th}}$ and $84^{\rm{th}}$ percentiles. The pink line shows the best power function fit to the data. While most of the points lie on the bottom left corner, around $5\%$ of them have $\rm{\tilde{R}}>0.4$ and $f_{\rm{esc}}>0.2$, and almost $20\%$ of the galaxies in the simulations contribute to these strongly leaking points.