The VANDELS survey: a strong correlation between Ly α equivalent width and stellar metallicity at 3 ≤ z ≤ 5

We present the results of a new study investigating the relationship between observed Ly α equivalent width (W_λ(Ly α)) and the metallicity of the ionizing stellar population ( Z_⋆) for a sample of 768 star-forming galaxies at 3 ≤ z ≤ 5 drawn from the VANDELS survey. Dividing our sample into quartiles of rest-frame W_λ(Ly α) across the range $-58 \,\rm {\mathring{\rm A}} \lesssim$ W_λ(Ly α) $\lesssim 110 \,\rm {\mathring{\rm A}}$ , we determine Z_⋆ from full spectral fitting of composite far-ultraviolet spectra and find a clear anticorrelation between W_λ(Ly α) and Z_⋆. Our results indicate that Z_⋆ decreases by a factor ≳ 3 between the lowest W_λ(Ly α) quartile (<W_λ(Ly α) $\rangle =-18\,\rm {\mathring{\rm A}}$ ) and the highest W_λ(Ly α) quartile (<W_λ(Ly α) $\rangle =24\,\rm {\mathring{\rm A}}$ ). Similarly, galaxies typically defined as Lyman alpha emitters (LAEs; W_λ(Ly α) $\gt 20\,\rm {\mathring{\rm A}}$ ) are, on average, metal poor with respect to the non-LAE galaxy population (W_λ(Ly α) $\le 20\,\rm {\mathring{\rm A}}$ ) with Z_⋆non-LAE ≳ 2 × Z_⋆LAE. Finally, based on the best-fitting stellar models, we estimate that the increasing strength of the stellar ionizing spectrum towards lower Z_⋆ is responsible for ${\simeq}15{-}25{{\ \rm per\ cent}}$ of the observed variation in W_λ(Ly α) across our sample, with the remaining contribution ( ${\simeq}75{-}85{{\ \rm per\ cent}}$ ) being due to a decrease in the H I/dust covering fractions in low- Z_⋆ galaxies.