The VANDELS survey: the ionizing properties of star-forming galaxies at $3 \leq z \leq 5$ using deep rest-frame ultraviolet spectroscopy

TL;DR

This study analyzes deep UV spectra of approximately 500 star-forming galaxies at redshifts 3 to 5 from the VANDELS survey to understand their ionizing photon escape and production efficiencies, shedding light on their role during the Epoch of Reionization.

Contribution

It provides new measurements of ionizing escape fractions and efficiencies for high-redshift galaxies, highlighting the properties of potential Lyman Continuum Emitters and their significance in cosmic reionization.

Findings

Higher escape fractions and ionizing efficiencies in low-mass, blue, Lyα-emitting galaxies.

Potential LCEs exhibit very blue UV slopes and strong Lyα and nebular lines.

UV-faint, low-mass, dustless galaxies likely dominated the ionizing budget during the EoR.

Abstract

To better understand the ionizing properties of galaxies in the EoR, we investigate deep, rest-frame ultraviolet (UV) spectra of $\simeq 500$ star-forming galaxies at $3 \leq z \leq 5$ selected from the public ESO-VANDELS spectroscopic survey. The absolute ionizing photon escape fraction ($f_{\rm esc}^{\rm abs}$) is derived by combining absorption line measurements with estimates of the UV attenuation. The ionizing production efficiency ($\xi_{ion}$) is calculated by fitting the far-UV (FUV) stellar continuum of the VANDELS galaxies. We find that the $f_{\rm esc}^{\rm abs}$ and $\xi_{ion}$ parameters increase towards low-mass, blue UV-continuum slopes and strong Ly$\alpha$ emitting galaxies, and both are just slightly higher-than-average for the UV-faintest galaxies in the sample. Potential Lyman Continuum Emitters (LCEs) and selected Lyman Alpha Emitters (LAEs) show systematically higher $\xi_{ion}$ ($\log \xi_{ion}$ (Hz\erg) $\approx 25.38, 25.41$) than non-LCEs and non-LAEs ($\log \xi_{ion}$ (Hz\erg) $\approx 25.18, 25.14$) at similar UV magnitudes. This indicates very young underlying stellar populations ($\approx 10~{\rm Myr}$) at relatively low metallicities ($\approx 0.2~{\rm Z_{\odot}}$). The FUV non-ionizing spectra of potential LCEs is characterized by very blue UV slopes ($\leq -2$), enhanced Ly$\alpha$ emission ($\leq -25$A), strong UV nebular lines (e.g., high CIV1550/CIII]1908 $\geq 0.75$ ratios), and weak absorption lines ($\leq 1$A). The latter suggests the existence of low gas-column-density channels in the interstellar medium which enables the escape of ionizing photons. By comparing our VANDELS results against other surveys in the literature, our findings imply that the ionizing budget in the EoR was likely dominated by UV-faint, low-mass and dustless galaxies.