A Search For C Ii 158 Μm Line Emission In Hcm 6a, A Lyα Emitter At Z = 6.56

Abstract

We report a Plateau de Bure interferometer search for CII-158μm emission from HCM6A, a lensed Lyman-α emitter (LAE) at z=6.56. Our non-detections of CII-158μm line emission and 1.2mm radio continuum emission yield 3σ limits of L_CII<6.4×10⁷ ×(ΔV/100kms^-1 )^1/2 L_⊙ on the CII-158μm line luminosity and S1.2mm<0.68 mJy on the 1.2mm flux density. The local conversion factor between LCII and star formation rate (SFR) yields an SFR <4.7 M⊙ yr−1, ≈2 times lower than that inferred from the ultraviolet (UV) continuum, suggesting that the local factor may not be applicable in high-z LAEs. The non-detection of 1.2mm continuum emission yields a total SFR <28 M_⊙/yr; any obscured star formation is thus within a factor of two of the visible star formation. Our best-fit model to the rest-frame UV/optical spectral energy distribution of HCM6A yields a stellar mass of 1.3×109 M_⊙ and an SFR of ~10 M_⊙/yr, with negligible dust obscuration. We fortuitously detect CO J=3-2 emission from a z=0.375 galaxy in the foreground cluster Abell370, obtaining a CO line luminosity of L′(CO)>(8.95±0.79)×108 K km s−1 pc2, and a molecular gas mass of M(H2)>(4.12±0.36)×109 M_⊙, for a CO-to-H2 conversion factor of 4.6 M⊙ (K km s−1 pc2)−1.