The VANDELS survey: a measurement of the average Lyman-continuum escape fraction of star-forming galaxies at z = 3.5
Schaerer, D.; Cullen, F.; McLure, R. J.; Dunlop, J. S.; Pentericci, L.; Fynbo, J. P. U.; Fontana, A.; Vanzella, E.; Zamorani, G.; Carnall, A. C.; McLeod, D. J.; Begley, R.; Hall, A.; Hamadouche, M. L.; Amorín, R.; Calabrò, A.; Guaita, L.; Hathi, N. P.; Hibon, P.; Ji, Z.; Llerena, M.; Saldana-Lopez, A.; Talia, M.
United Kingdom, Chile, Italy, Denmark, United States, Switzerland
Abstract
We present a study designed to measure the average Lyman-continuum escape fraction (⟨fesc⟩) of star-forming galaxies at z ≃ 3.5. We assemble a sample of 148 galaxies from the VANDELS spectroscopic survey at 3.35 ≤ zspec ≤ 3.95, selected to minimize line-of-sight contamination of their photometry. For this sample, we use ultra-deep, ground-based, U-band imaging and Hubble Space Telescope V-band imaging to robustly measure the distribution of $\mathcal {R_{\rm obs}}\, =(L_{\rm LyC}/L_{\rm UV})_{\rm obs}$. We then model the $\mathcal {R_{\rm obs}}$ distribution as a function of ⟨fesc⟩, carefully accounting for attenuation by dust, the intergalactic medium and the circumgalactic medium. A maximum likelihood fit to the $\mathcal {R_{\rm obs}}$ distribution returns a best-fitting value of $\langle f_{\rm esc}\rangle =0.07^{+0.02}_{-0.02}$, a result confirmed using an alternative Bayesian inference technique (both techniques exclude ⟨fesc⟩ = 0.0 at >3σ). By splitting our sample in two, we find evidence that ⟨fesc⟩ is positively correlated with Ly α equivalent width (Wλ(Ly α)), with high and low Wλ(Lyα) subsamples returning values of $\langle f_{\rm esc}\rangle =0.12^{+0.06}_{-0.04}$ and $\langle f_{\rm esc} \rangle =0.02^{+0.02}_{-0.01}$, respectively. In contrast, we find evidence that ⟨fesc⟩ is anticorrelated with intrinsic UV luminosity and UV dust attenuation; with low UV luminosity and dust attenuation subsamples both returning best fits in the range 0.10 ≤ ⟨fesc⟩ ≤ 0.22. We do not find a clear correlation between fesc and galaxy stellar mass, suggesting stellar mass is not a primary indicator of fesc. Although larger samples are needed to further explore these trends, our results suggest that it is entirely plausible that the low dust, low-metallicity galaxies found at z ≥ 6 will display the ⟨fesc⟩ ≥ 0.1 required to drive reionization.