We present the results of ALMA spectroscopic follow-up of a $z=6.765$ Lyman-$\alpha$ emitting galaxy behind the cluster RXJ1347-1145. We report the detection of [CII]158$\mu$m line fully consistent with the Lyman-$\alpha$ redshift and with the peak of the optical emission. Given the magnification of $\mu=5.0 \pm 0.3$ the intrinsic (corrected for lensing) luminosity of the [CII] line is $L_{ [CII]} =1.4^{ +0.2}_{ -0.3} \times 10^7L_{ \odot}$, which is ${ \sim}5$ times fainter than other detections of $z\sim 7$ galaxies. The result indicates that low $L_{ [CII]}$ in $z\sim 7$ galaxies compared to the local counterparts might be caused by their low metallicities and/or feedback. The small velocity off-set ($\Delta v = 20_{ -40}^{ +140} \rm km/s$) between the Lyman-$\alpha$ and [CII] line is unusual, and may be indicative of ionizing photons escaping.
Categories: astro-ph.GA astro-ph.CO