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 Lyman-continuum escape fraction (< f(esc)>) of star-forming galaxies at similar or equal to 3.5. We assemble a sample of 148 galaxies from the VANDELS spectroscopic survey at 3.35 <= z(spec) <= 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 R-obs = (L-LyC/L-UV)(obs). We then model the R-obs distribution as a function of < f(esc)>, carefully accounting for attenuation by dust, the intergalactic medium and the circumgalactic medium. A maximum likelihood fit to the R(obs )distribution returns a best-fitting value of < f(esc)> = 0.07(-0.02)(+0.02), a result confirmed using an alternative Bayesian inference technique (both techniques exclude < f(esc)> = 0.0 at >3 sigma). By splitting our sample in two, we find evidence that < f(esc)> is positively correlated with Ly a equivalent width (W-lambda(Ly alpha)), with high and low W-lambda(Ly alpha) subsamples returning values of < f(esc)> = 0.12(-0.04)(+0.06) and < f(esc)> = 0.02(-0.01)(+0.02), respectively. In contrast, we find evidence that < f(esc)> 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 <= < f(esc)> <= 0.22. We do not find a clear correlation between f(esc) and galaxy stellar mass, suggesting stellar mass is not a primary indicator of f(esc). 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 < f(esc)> >= 0.1 required to drive reionization.