Connecting the escape fraction of Lyman-alpha and Lyman-continuum photons in star-forming galaxies at z ≃ 4–5

ABSTRACT We present a study of the connection between the escape fraction of Lyman-alpha (Ly\, ) and Lyman-continuum (LyC) photons within a sample of N = 152 star-forming galaxies selected from the VANDELS survey at 3. 85 ≤ zspec ≤ 4. 95 (〈zspec〉 = 4. 36). By combining measurements of H\, equivalent width (W (H\, ) ) derived from broad-band photometry with measurements of Ly\, equivalent width (W (Ly\, ) ) from the VANDELS spectra, we individually estimate f ₄ₒ₂^ {Ly } for our full sample. In agreement with previous studies, we find a positive correlation between W (Ly\, ) and f ₄ₒ₂^ {Ly }, with f ₄ₒ₂^ {Ly } increasing from f ₄ₒ₂^ {Ly } 0. 04 at W (Ly\, ) =10 Å to f ₄ₒ₂^ {Ly } 0. 1 at W (Ly\, ) =25 Å. For the first time at z ≃ 4–5, we investigate the relationship between f ₄ₒ₂^ {Ly } and f ₄ₒ₂^ {LyC} using f ₄ₒ₂^ {LyC} estimates derived using the equivalent widths of low-ionization, far-ultraviolet absorption lines in composite VANDELS spectra. Our results indicate that f ₄ₒ₂^ {LyC} rises monotonically with f ₄ₒ₂^ {Ly }, following a relation of the form f ₄ₒ₂^ {LyC} 0. 15^+0. 06-₀. ₀₄f ₄ₒ₂^ {Ly }. Based on composite spectra of sub-samples with approximately constant Wλ (Lyα), but widely different f ₄ₒ₂^ {Ly }, we demonstrate that the f ₄ₒ₂^ {LyC}−f ₄ₒ₂^ {Ly }correlation is not driven by a secondary correlation between f ₄ₒ₂^ {Ly } and Wλ (Lyα). The observed f ₄ₒ₂^ {LyC}−f ₄ₒ₂^ {Ly } correlation is in good qualitative agreement with theoretical predictions and provides further evidence that estimates of f ₄ₒ₂^ {LyC} within the Epoch of Reionization should be based on proxies sensitive to neutral gas density/geometry and dust attenuation.