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× {10}7 {L}⊙ , 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 ({{Δ }}v={20}-40+140 {km} {{{s}}}-1) between the Lyα and [C II] line is unusual, and may be indicative of ionizing photons escaping.