Alma observation of 158μm [Cii] line and dust continuum of a z = 7 normally star-forming galaxy in the epoch of reionization

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 σline = 240 μJy beam^-1 (40 km s^-1 channel) and σ_cont = 21 μJy beam^-1, 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 < 6.4 × 10⁷ M_⊙, FIR luminosity L_⊙FIR < 3.7 × 10¹⁰ L_⊙ (42.5-122.5 μm), total IR luminosity L_⊙IR < 5.7 × 10¹⁰ L _⊙ (8-1000 μm), and dust-obscured star formation rate (SFR) < 10 M_⊙ yr^-1, 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] < 3.4 × 10⁷ L_⊙. Locations of IOK-1 and previously observed LAEs on the L_⊙[C II] versus SFR and L _⊙[C II]/L_⊙FIR versus L_⊙FIR diagrams 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.