MMT spectroscopy of lyman-alpha at z≃7: Evidence for accelerated reionization around massive galaxies

Reionization-era galaxies tend to exhibit weak Lyα emission, likely reflecting attenuation from an increasingly neutral IGM. Recent observations have begun to reveal exceptions to this picture, with strong Lyα emission now known in four of the most massive z=7–9 galaxies in the CANDELS fields, all of which also exhibit intense [OIII]+Hβ emission (EW>800 Å). To better understand why Lyα is anomalously strong in a subset of massive z 7 − 9 galaxies, we have initiated an MMT/Binospec survey targeting a larger sample (N=22) of similarly luminous ('1–6 L^∗_UV) z'7 galaxies selected over very wide-area fields (∼3 deg²). We confidently (>7σ) detect Lyα in 78% (7/9) of galaxies with strong [OIII]+Hβ emission (EW>800 Å) as opposed to only 8% (1/12) of galaxies with more moderate (EW=200-800 Å) [OIII]+Hβ. We argue that the higher Lyα EWs of the strong [OIII]+Hβ population likely reflect enhanced ionizing photon production efficiency owing to their large sSFRs (&30 Gyr⁻¹). We also find evidence that Lyα transmission from massive galaxies declines less rapidly over 6 < z < 7 than in low-mass lensed systems. In particular, our data suggest no strong evolution in Lyα transmission, consistent with a picture wherein massive z'7 galaxies often reside in large ionized regions. We detect three closely-separated (R = 1.7 physical Mpc) z'7 Lyα emitters in our sample, conceivably tracing a large ionized structure that is consistent with this picture. We detect tentative evidence for an overdensity in this region, implying a large ionizing photon budget in the surrounding volume.