Abstract

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 Ly alpha emitter (LAE) IOK-1. Probing to sensitivities of sigma(line) = 240 mu Jy beam(-1) (40 km s(-1) channel) and sigma(cont) = 21 mu Jy 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 3 sigma FIR continuum limit, corrected for CMB effects, implies intrinsic dust mass M-dust < 6.4 x 10(7) M-circle dot, FIR luminosity L-FIR < 3.7 x 10(10) L-circle dot (42.5-122.5 mu m), total IR luminosity L-IR < 5.7 x 10(10) L-circle dot (8-1000 mu m), and dust-obscured star formation rate (SFR) < 10 M-circle dot 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 3 sigma [C II] flux limit translates into [C II] luminosity, L-[C II] < 3.4 x 10(7) L-circle dot. 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.