The VANDELS survey: a measurement of the average Lyman-continuum escape fraction of star-forming galaxies at z=3.5

We present a study designed to measure the average LyC escape fraction (⟨fesc⟩) of star-forming galaxies at z=3.5. We assemble a sample of 148 galaxies from the VANDELS 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 HST V−band imaging to robustly measure the distribution of Robs =(LLyC/LUV)obs. We then model the distribution as a function of ⟨fesc⟩, carefully accounting for attenuation by dust, and the IGM (and CGM). A maximum likelihood fit to the Robs distribution returns a best-fitting value of ⟨fesc⟩=0.07±0.02, a result confirmed using an alternative Bayesian inference technique (both exclude ⟨fesc⟩=0.0 at >3σ). By splitting our sample in two, we find evidence that ⟨fesc⟩ is positively correlated with Lyα equivalent width, with high and low sub-samples returning best fits of ⟨fesc⟩=0.12+0.06−0.04 and ⟨fesc⟩=0.02+0.02−0.01, respectively. In contrast, we find evidence that ⟨fesc⟩ is anti-correlated with intrinsic UV luminosity and UV dust attenuation; with low UV luminosity and dust attenuation sub-samples returning best fits in the range 0.10≤⟨fesc⟩≤0.22. We do not find evidence for a clear correlation between fesc and galaxy stellar mass, suggesting it is not a primary indicator of leakage. Although larger samples are needed to robustly confirm these trends, they suggest that it is entirely plausible that the low dust and metallicity galaxies found at z > 6 will display the ⟨fesc⟩≥0.1 required to drive reionization.