Abstract
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 \ctf 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
$L_CII =1.4^+0.2_-0.3 10^7L_ødot$, which is $\sim5$ times
fainter than other detections of $z7$ galaxies. The result indicates that
low $L_CII$ in $z7$ galaxies compared to the local counterparts are
likely caused by their low metallicities and/or feedback. The small velocity
off-set ($\Delta v = 20_-40^+140km/s$) between the Lyman-$\alpha$
and CII line is unusual, and may be indicative of ionizing photons escaping.
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