The VANDELS survey: A strong correlation between Ly$\alpha$ equivalent width and stellar metallicity at $3\le \mathbf{z}\le 5$

Abstract

We present the results of a new study investigating the relationship between observed Ly$\alpha$ equivalent width ($W_{\lambda }$(Ly$\alpha$)) and the metallicity of the ionizing stellar population ($Z_{\star }$) for a sample of $768$ star-forming galaxies at $3\le z\le 5$ drawn from the VANDELS survey. Dividing our sample into quartiles of rest-frame $W_{\lambda }$(Ly$\alpha$) across the range $-58 \unicode{xC5} \lesssim$ $W_{\lambda }$(Ly$\alpha$) $\lesssim 110 \unicode{xC5}$ we determine $Z_{\star }$ from full spectral fitting of composite far-ultraviolet (FUV) spectra and find a clear anti-correlation between $W_{\lambda }$(Ly$\alpha$) and $Z_{\star }$. Our results indicate that $Z_{\star }$ decreases by a factor $\gtrsim 3$ between the lowest $W_{\lambda }$(Ly$\alpha$) quartile ($⟨$$W_{\lambda }$(Ly$\alpha$)$\rangle=-18\unicode{xC5}$) and the highest $W_{\lambda }$(Ly$\alpha$) quartile ($⟨$$W_{\lambda }$(Ly$\alpha$)$\rangle=24\unicode{xC5}$). Similarly, galaxies typically defined as Lyman Alpha Emitters (LAEs; $W_{\lambda }$(Ly$\alpha$) $>20\unicode{xC5}$) are, on average, metal poor with respect to the non-LAE galaxy population ($W_{\lambda }$(Ly$\alpha$) $\leq20\unicode{xC5}$) with $Z_{\star }$${}_{\mathrm{non}-\mathrm{LAE}}\gtrsim 2\times$ $Z_{\star }$${}_{\mathrm{LAE}}$. Finally, based on the best-fitting stellar models, we estimate that the increasing strength of the stellar ionizing spectrum towards lower $Z_{\star }$ is responsible for $\simeq 15-25\%$ of the observed variation in $W_{\lambda }$(Ly$\alpha$) across our sample, with the remaining contribution ($\simeq 75-85\%$) being due to a decrease in the HI/dust covering fractions in low $Z_{\star }$ galaxies.

arXiv comment10 pages, 6 figures, MNRAS accepted

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