A search for CII-158$μ$m line emission in HCM\,6A, a Lyman-$α$ emitter at $z=6.56$

Abstract: We report a Plateau de Bure interferometer search for CII-158$\mu$m emission from HCM6A, a lensed Lyman-$\alpha$ emitter (LAE) at $z = 6.56$. Our non-detections of CII-158$\mu$m line emission and 1.2mm radio continuum emission yield $3\sigma$ limits of L${\rm CII} < 6.4 \times 10^7 \times (\Delta V/100 km s^{-1})^{1/2}$ L$\odot$ on the CII-158$\mu$m line luminosity and S${\rm 1.2mm} < 0.68$ mJy on the 1.2mm flux density. The local conversion factor between L${\rm CII}$ and star formation rate (SFR) yields an SFR $< 4.7$ M$\odot$ yr$^{-1}$, $\approx 2$ 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$\odot$/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 $1.3 \times 10^9$ M$\odot$ and an SFR of ~10 M$\odot$/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$^\prime ({\rm CO}) > (8.95 \pm 0.79) \times 10^8$ K km s$^{-1}$ pc$^2$, and a molecular gas mass of M$({\rm H_2}) > (4.12 \pm 0.36) \times 10^9$ M$\odot$, for a CO-to-H$_2$ conversion factor of 4.6 M$\odot$ (K km s$^{-1}$ pc$^2$)$^{-1}$.