ALMA [C ii] 158 μm detection of a redshift 7 lensed galaxy behind RX J1347.1-1145

We present the results of ALMA spectroscopic follow-up of a z = 6.766 Lyα emitting galaxy behind the cluster RX J1347.1−1145. We report the detection of [C ii] 158 μm line fully consistent with the Lyα redshift and with the peak of the optical emission. Given the magnification of μ = 5.0 ± 0.3, the intrinsic (corrected for lensing) luminosity of the [C ii] line is L _{[C ii]} $={1.4}_{-0.3}^{+0.2}\times {10}^7{L}_{\odot }$, roughly ∼5 times fainter than other detections of z ∼ 7 galaxies. The result indicates that low L _{[C ii]} in z ∼ 7 galaxies compared to the local counterparts might be caused by their low metallicities and/or feedback. The small velocity offset ($\Delta v={20}_{-40}^{+140}km{s}^{-1}$) between the Lyα and [C ii] line is unusual, and may be indicative of ionizing photons escaping.