2014ApJ...792...34O

We present ALMA observations of the [C II] line and far-infrared (FIR) continuum of a normally star-forming galaxy in the reionization epoch, the z = 6.96 Lyα emitter (LAE) IOK-1. Probing to sensitivities of σ_line= 240 µJy/beam (40 km/s channel) and σ_cont= 21 µJy/beam, we found the galaxy undetected in both [C II] and continuum. Comparison of ultraviolet (UV)-FIR spectral energy distribution (SED) of IOK-1, including our ALMA limit, with those of several types of local galaxies (including the effects of the cosmic microwave background, CMB, on the FIR continuum) suggests that IOK-1 is similar to local dwarf/irregular galaxies in SED shape rather than highly dusty/obscured galaxies. Moreover, our 3σ FIR continuum limit, corrected for CMB effects, implies intrinsic dust mass M_dust< 6.4x10⁷ M_☉, FIR luminosity L_FIR< 3.7x10¹⁰ L_☉(42.5-122.5 µm), total IR luminosity L_IR< 5.7x10¹⁰ L_☉(8-1000 µm), and dust-obscured star formation rate (SFR) < 10 M_☉/yr, if we assume that IOK-1 has a dust temperature and emissivity index typical of local dwarf galaxies. This SFR is 2.4 times lower than one estimated from the UV continuum, suggesting that <29% of the star formation is obscured by dust. Meanwhile, our 3σ [C II] flux limit translates into [C II] luminosity, L_[C II]_< 3.4x10⁷ L_☉. Locations of IOK-1 and previously observed LAEs on the L_[C II]_ versus SFR and L_[C II]_/LFIR_versus L_FIRdiagrams imply that LAEs in the reionization epoch have significantly lower gas and dust enrichment than AGN-powered systems and starbursts at similar/lower redshifts, as well as local star-forming galaxies.