Connecting the escape fraction of Lyman-alpha and Lyman-continuum photons in star-forming galaxies at $\mathbf{z\simeq 4-5}$

Abstract

We present a study of the connection between the escape fraction of Lyman-alpha (Ly$α$) and Lyman-continuum (LyC) photons within a sample of N=152 star-forming galaxies selected from the VANDELS survey at $3.85<z_{spec}<4.95$. By combining measurements of H$α$ equivalent width $(W_λ(\rm{Hα}))$ derived from broad-band photometry with Ly$α$ equivalent width $(W_λ(Lyα))$ measurements from VANDELS spectra, we individually estimate $f_{\rm{esc}}^{Lyα}$ for our full sample. In agreement with previous studies, we find a positive correlation between $W_λ(Lyα)$ and $f_{\rm{esc}}^{Lyα}$, increasing from $f_{\rm{esc}}^{Lyα}\simeq0.04$ at $W_λ(Lyα)=10$Å to $f_{\rm{esc}}^{Lyα}\simeq0.1$ at $W_λ(Lyα)=25$Å. For the first time at $z\sim4-5$, we investigate the relationship between $f_{\rm{esc}}^{Lyα}$ and $f_{\rm{esc}}^{\rm{LyC}}$ using $f_{\rm{esc}}^{\rm{LyC}}$ estimates derived using the equivalent widths of low-ionization, FUV absorption lines in composite VANDELS spectra. Our results indicate that $f_{\rm{esc}}^{\rm{LyC}}$ rises monotonically with $f_{\rm{esc}}^{Lyα}$, following the relation $f_{\rm{esc}}^{\rm{LyC}}\simeq 0.15^{+0.06}_{-0.04}f_{\rm{esc}}^{Lyα}$. Based on composite spectra of sub-samples with roughly constant $W_λ(Lyα)$, but very different $f_{\rm{esc}}^{Lyα}$, we show that the $f_{\rm{esc}}^{\rm{LyC}}-f_{\rm{esc}}^{Lyα}$ correlation is not driven by a secondary correlation between $f_{\rm{esc}}^{Lyα}$and $W_λ(Lyα)$. The $f_{\rm{esc}}^{\rm{LyC}}-f_{\rm{esc}}^{Lyα}$ correlation is in good qualitative agreement with theoretical predictions and provides further evidence that estimates of $f_{\rm{esc}}^{\rm{LyC}}$ within the Epoch of Reionization should be based on proxies sensitive to neutral gas density/geometry and dust attenuation.