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

Abstract

We present the results of a new study investigating the relationship between observed Ly α equivalent width (W<SUB>λ</SUB>(Ly α)) and the metallicity of the ionizing stellar population ( Z<SUB>⋆</SUB>) 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<SUB>λ</SUB>(Ly α) across the range $-58 \,\rm {\mathring{\rm A}} \lesssim$ W<SUB>λ</SUB>(Ly α) $\lesssim 110 \,\rm {\mathring{\rm A}}$ , we determine Z<SUB>⋆</SUB> from full spectral fitting of composite far-ultraviolet spectra and find a clear anticorrelation between W<SUB>λ</SUB>(Ly α) and Z<SUB>⋆</SUB>. Our results indicate that Z<SUB>⋆</SUB> decreases by a factor ≳ 3 between the lowest W<SUB>λ</SUB>(Ly α) quartile (<W<SUB>λ</SUB>(Ly α) $\rangle =-18\,\rm {\mathring{\rm A}}$ ) and the highest W<SUB>λ</SUB>(Ly α) quartile (<W<SUB>λ</SUB>(Ly α) $\rangle =24\,\rm {\mathring{\rm A}}$ ). Similarly, galaxies typically defined as Lyman alpha emitters (LAEs; W<SUB>λ</SUB>(Ly α) $\gt 20\,\rm {\mathring{\rm A}}$ ) are, on average, metal poor with respect to the non-LAE galaxy population (W<SUB>λ</SUB>(Ly α) $\le 20\,\rm {\mathring{\rm A}}$ ) with Z<SUB>⋆</SUB><SUB>non-LAE</SUB> ≳ 2 × Z<SUB>⋆</SUB><SUB>LAE</SUB>. Finally, based on the best-fitting stellar models, we estimate that the increasing strength of the stellar ionizing spectrum towards lower Z<SUB>⋆</SUB> is responsible for ${\simeq}15{-}25{{\ \rm per\ cent}}$ of the observed variation in W<SUB>λ</SUB>(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<SUB>⋆</SUB> galaxies.