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$\alpha$) 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$\alpha$ equivalent width $(W_{\lambda }(\mathrm{H}\alpha ))$ derived from broad-band photometry with Ly$\alpha$ equivalent width $(W_{\lambda }(Ly\alpha ))$ measurements from VANDELS spectra, we individually estimate $f_{\mathrm{esc}}^{Ly\alpha }$ for our full sample. In agreement with previous studies, we find a positive correlation between $W_{\lambda }(Ly\alpha )$ and $f_{\mathrm{esc}}^{Ly\alpha }$, increasing from $f_{\mathrm{esc}}^{Ly\alpha }\simeq 0.04$ at $W_{\lambda }(Ly\alpha )=10$Å to $f_{\mathrm{esc}}^{Ly\alpha }\simeq 0.1$ at $W_{\lambda }(Ly\alpha )=25$Å. For the first time at $z\sim 4-5$, we investigate the relationship between $f_{\mathrm{esc}}^{Ly\alpha }$ and $f_{\mathrm{esc}}^{\mathrm{LyC}}$ using $f_{\mathrm{esc}}^{\mathrm{LyC}}$ estimates derived using the equivalent widths of low-ionization, FUV absorption lines in composite VANDELS spectra. Our results indicate that $f_{\mathrm{esc}}^{\mathrm{LyC}}$ rises monotonically with $f_{\mathrm{esc}}^{Ly\alpha }$, following the relation $f_{\mathrm{esc}}^{\mathrm{LyC}}\simeq 0.15_{-0.04}^{+0.06}{f}_{\mathrm{esc}}^{Ly\alpha }$. Based on composite spectra of sub-samples with roughly constant $W_{\lambda }(Ly\alpha )$, but very different $f_{\mathrm{esc}}^{Ly\alpha }$, we show that the $f_{\mathrm{esc}}^{\mathrm{LyC}}-f_{\mathrm{esc}}^{Ly\alpha }$ correlation is not driven by a secondary correlation between $f_{\mathrm{esc}}^{Ly\alpha }$and $W_{\lambda }(Ly\alpha )$. The $f_{\mathrm{esc}}^{\mathrm{LyC}}-f_{\mathrm{esc}}^{Ly\alpha }$ correlation is in good qualitative agreement with theoretical predictions and provides further evidence that estimates of $f_{\mathrm{esc}}^{\mathrm{LyC}}$ within the Epoch of Reionization should be based on proxies sensitive to neutral gas density/geometry and dust attenuation.

12 pages, 7 figures, Accepted for publication in Monthly Notices of the Royal Astronomical Society