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

Abstract

We present a study designed to measure the average LyC escape fraction ($⟨f_{\mathrm{esc}}⟩$) of star-forming galaxies at z=3.5. We assemble a sample of 148 galaxies from the VANDELS survey at $3.35\le z_{\mathrm{spec}}\le 3.95$, 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 ${\mathcal{R}}_{\mathrm{obs}}$ $=(L_{\mathrm{LyC}}/L_{\mathrm{UV}}{)}_{\mathrm{obs}}$. We then model the distribution as a function of $⟨f_{\mathrm{esc}}⟩$, carefully accounting for attenuation by dust, and the IGM (and CGM). A maximum likelihood fit to the ${\mathcal{R}}_{\mathrm{obs}}$ distribution returns a best-fitting value of $⟨f_{\mathrm{esc}}⟩=0.07\pm 0.02$, a result confirmed using an alternative Bayesian inference technique (both exclude $⟨f_{\mathrm{esc}}⟩=0.0$ at $>3\ddot{I}ƒ$). By splitting our sample in two, we find evidence that $⟨f_{\mathrm{esc}}⟩$ is positively correlated with Ly$\hat{I}\pm$ equivalent width, with high and low sub-samples returning best fits of $⟨f_{\mathrm{esc}}⟩=0.12_{-0.04}^{+0.06}$ and $⟨f_{\mathrm{esc}}⟩=0.02_{-0.01}^{+0.02}$, respectively. In contrast, we find evidence that $⟨f_{\mathrm{esc}}⟩$ 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 $0.10\le ⟨f_{\mathrm{esc}}⟩\le 0.22$. We do not find evidence for a clear correlation between $f_{\mathrm{esc}}$ and galaxy stellar mass, suggesting it is not a primary indicator of leakage. Although larger samples are needed to further explore these trends, they suggest that it is entirely plausible that the low dust and metallicity galaxies found at z > 6 will display the $⟨f_{\mathrm{esc}}⟩\ge 0.1$ required to drive reionization.

15 pages (+ appendix), 12 figures, 1 table, Accepted for publication in Monthly Notices of the Royal Astronomical Society