The archival discovery of a strong Lyman-$\alpha$ and [CII] emitter at z = 7.677

Abstract

We report the archival discovery of Lyman-$\alpha$ emission from the bright ultraviolet galaxy Y002 at $z=7.677$, spectroscopically confirmed by its ionized carbon [CII] 158$\mu$m emission line. The Ly$\alpha$ line is spatially associated with the rest-frame UV stellar emission ($M_{\mathrm{UV}}$~-22, 2x brighter than $M_{\mathrm{UV}}^{\star }$) and it appears offset from the peak of the extended [CII] emission at the current ~1" spatial resolution. We derive an estimate of the unobscured SFR(UV)=$(22\pm 1)M_{\odot }$ yr${}^{-1}$ and set an upper limit of SFR(IR)$<15M_{\odot }$ yr${}^{-1}$ from the far-infrared wavelength range, which globally place Y002 on the SFR(UV+IR)-L([CII]) correlation observed at lower redshifts. In terms of velocity, the peak of the Ly$\alpha$ emission is redshifted by $\Delta v$(Ly$\alpha$)~500 km s${}^{-1}$ from the systemic redshift set by [CII] and a high-velocity tail extends to up to ~1000 km s${}^{-1}$. The velocity offset is up to ~3.5x 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$\alpha$ light, as indicated by the large equivalent width EW(Ly$\alpha$)=$24_{-6}^{+5}$ Å. Assuming that [CII] traces the neutral hydrogen, we estimate a HI gas fraction of $M(\mathrm{HI})/M_{\star }\gtrsim 8$ for Y002 as a system and speculate that patches of high HI column densities could contribute to explain the observed spatial offsets between Ly$\alpha$ and [CII] emitting regions. The low dust content, implied by the non-detection of the far-infrared continuum emission at rest-frame ~160 $\mu$m, would be sufficient to absorb any potential Ly$\alpha$ photons produced within the [CII] clump as a result of large HI column densities.

arXiv comment10 pages, 4 figures. Accepted for publication in The Astrophysical Journal Letters

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