Lyman-alpha spectroscopy of extreme [OIII] emitting galaxies at $z\simeq 2-3$: Implications for Ly$\hat{I}\pm$ visibility and LyC leakage at $z>6$

Abstract

Spectroscopic observations of massive $z>7$ galaxies selected to have extremely large [OIII]+H${\hat{I}}^2$ equivalent width (EW $\sim 1500$ Ã ) have recently revealed large Ly$\hat{I}\pm$ detection rates, in contrast to the weak emission seen in the general population. Why these systems are uniquely visible in Ly$\hat{I}\pm$ at redshifts where the IGM is likely significantly neutral is not clear. With the goal of better understanding these results, we have begun a campaign with MMT and Magellan to measure Ly$\hat{I}\pm$ in galaxies with similar [OIII]+H${\hat{I}}^2$ EWs at $z\simeq 2-3$. At these redshifts, the IGM is highly ionized, allowing us to clearly disentangle how the Ly$\hat{I}\pm$ properties depend on the [OIII]+H${\hat{I}}^2$ EW. Here we present Ly$\hat{I}\pm$ EWs of $49$ galaxies at $z=2.2-3.7$ with intense [OIII]+H${\hat{I}}^2$ line emission (EW $=300-3000$ Ã ). Our results demonstrate that strong Ly$\hat{I}\pm$ emission (EW $>20$ Ã ) becomes more common in galaxies with larger [OIII]+H${\hat{I}}^2$ EW, reflecting a combination of increasingly efficient ionizing photon production and enhanced transmission of Ly$\hat{I}\pm$. Among the galaxies with the most extreme [OIII]+H${\hat{I}}^2$ emission (EW $\sim 1500$ Ã ), we find that strong Ly$\hat{I}\pm$ emission is not ubiquitous, with only $50$ per cent of our population showing Ly$\hat{I}\pm$ EW $>20$ Ã . Our data suggest that the range of Ly$\hat{I}\pm$ strengths is related to the observed ellipticity, with those systems that appear edge-on or elongated having weaker Ly$\hat{I}\pm$ emission. We use these results to interpret the anomalous Ly$\hat{I}\pm$ properties seen in $z>7$ galaxies with extreme [OIII]+H${\hat{I}}^2$ emission and discuss implications for the escape of ionizing radiation from these extreme line emitting galaxies.

13 pages, 8 figure, accepted by MNRAS