A search for CII-158$\hat{I}¼$m line emission in HCM\,6A, a Lyman-$\hat{I}\pm$ emitter at $z=6.56$

Abstract

We report a Plateau de Bure interferometer search for CII-158$\hat{I}¼$m emission from HCM6A, a lensed Lyman-$\hat{I}\pm$ emitter (LAE) at $z=6.56$. Our non-detections of CII-158$\hat{I}¼$m line emission and 1.2mm radio continuum emission yield $3\ddot{I}ƒ$ limits of L${}_{\mathrm{CII}}<6.4\times 10^7\times (\hat{I}”V/100km{s}^{-1}{)}^{1/2}$ L${}_{\odot }$ on the CII-158$\hat{I}¼$m line luminosity and S${}_{1.2mm}<0.68$ mJy on the 1.2mm flux density. The local conversion factor between L${}_{\mathrm{CII}}$ 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${}^{\prime }(\mathrm{CO})>(8.95\pm 0.79)\times 10^8$ K km s${}^{-1}$ pc${}^2$, and a molecular gas mass of M$({\mathrm{H}}_2)>(4.12\pm 0.36)\times 10^9$ M${}_{\odot }$, for a CO-to-H${}_2$ conversion factor of 4.6 M${}_{\odot }$ (K km s${}^{-1}$ pc${}^2$)${}^{-1}$.

6 pages, 4 figures; accepted for publication in Astrophysical Journal Letters