The VANDELS survey: the ionizing properties of star-forming galaxies at 3z5 using deep rest-frame ultraviolet spectroscopy

A. Saldana-Lopez, D. Schaerer, J. Chisholm, A. Calabrò, L. Pentericci, A. Saxena, R. Amorín, A. C. Carnall, F. Fontanot, J. P. U. Fynbo, L. Guaita, N. P. Hathi, P. Hibon Z. Ji D. J. McLeod, E. Pompei, G. Zamorani

Submitted on 2 November 2022


To better understand the ionizing properties of galaxies in the EoR, we investigate deep, rest-frame ultraviolet (UV) spectra of 500 star-forming galaxies at 3z5 selected from the public ESO-VANDELS spectroscopic survey. The absolute ionizing photon escape fraction (fescabs) is derived by combining absorption line measurements with estimates of the UV attenuation. The ionizing production efficiency (ξion) is calculated by fitting the far-UV (FUV) stellar continuum of the VANDELS galaxies. We find that the fescabs and ξion parameters increase towards low-mass, blue UV-continuum slopes and strong Lyα 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 ξion (logξion (Hz\erg) 25.38,25.41) than non-LCEs and non-LAEs (logξion (Hz\erg) 25.18,25.14) at similar UV magnitudes. This indicates very young underlying stellar populations (10 Myr) at relatively low metallicities (0.2 Z). The FUV non-ionizing spectra of potential LCEs is characterized by very blue UV slopes (2), enhanced Lyα emission (25A), strong UV nebular lines (e.g., high CIV1550/CIII]1908 0.75 ratios), and weak absorption lines (1A). 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.


Comment: 25 pages, 18 figures, 2 tables; submitted to MNRAS

Subjects: Astrophysics - Astrophysics of Galaxies; Astrophysics - Cosmology and Nongalactic Astrophysics


{\bf Top:} \textsc{FiCUS} SED fitting results for the CDFS017345 VANDELS galaxy. The observed and error spectra are displayed in black and light-green. The best-fit stellar continuum is overplotted in red, while the spectral regions masked during the fit are shown in gray. The spectrum, shown in ${\rm F_{\lambda}}$ units, has been normalized over $1350-1370$\AA. The most prominent stellar features, nebular and ISM absorption lines are indicated with blue, gold and black vertical lines at the top part of the figure. {\bf Bottom left:} light-fractions as a function of age and metallicity for the best-fit composite stellar population model. {\bf Bottom right:} summary chart with the best-fit SED fitting parameters, i.e., reduced-$\chi^2$, \ebv\ (in mag., \citetalias{R16}), light-weighted stellar age (in Myr) and metallicity (in \Zsun).