The Archival Discovery of a Strong Lyα and [C ii] Emitter at z = 7.677

Abstract

We report the archival discovery of Ly α emission from the bright ultraviolet galaxy Y002 at z = 7.677, spectroscopically confirmed by its ionized carbon [C ii ] 158 μ m emission line. The Ly α line is spatially associated with the rest-frame UV stellar emission ( M UV ∼ −22, 2× brighter than M UV ⋆ ), and it appears offset from the peak of the extended [C ii ] emission at the current ∼1″ spatial resolution. We derive an estimate of the unobscured SFR UV = (22 ± 1) M ⊙ yr −1 and set an upper limit of SFR IR < 15 M ⊙ yr −1 from the far-infrared (FIR) wavelength range, which globally places Y002 on the SFR(UV+IR)– L [C II ] correlation observed at lower redshifts. In terms of velocity, the peak of the Ly α emission is redshifted by Δ v Ly α ∼ 500 km s −1 from the systemic redshift set by [C ii ] and a high-velocity tail extends up to ∼1000 km s −1 . The velocity offset is up to ∼3.5× higher than the average estimate for similarly UV-bright emitters at z ∼ 6–7, which might suggest that we are witnessing the merging of two clumps. A combination of strong outflows and the possible presence of an extended ionized bubble surrounding Y002 would likely facilitate the escape of copious Ly α light, as indicated by the large equivalent width EW 0 (Ly α ) = 24 − 6 + 5 Å. Assuming that [C ii ] traces the neutral hydrogen, we estimate a H i gas fraction of M H I / M ⋆ ≳ 8 for Y002 as a system and speculate that patches of high H i column densities could contribute to explaining the observed spatial offsets between Ly α- and [C ii ]-emitting regions. The low dust content, implied by the nondetection of the FIR continuum emission at rest frame ∼160 μ m, would be sufficient to absorb any potential Ly α photons produced within the [C ii ] clump as a result of large H i column densities.