2022MNRAS.513.3510B

We present a study designed to measure the average Lyman-continuum escape fraction (<f_esc> ) of star-forming galaxies at z ≃ 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 \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 <f_esc> , 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 <f_esc> = 0.0 at >3σ). By splitting our sample in two, we find evidence that <f_esc> is positively correlated with Ly α equivalent width (W_λ(Ly α)), with high and low W_λ(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 <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.