Spectroscopic confirmation of a gravitationally lensed Lyman break galaxy at z[C ii]= 6.827 using NOEMA

We present the spectroscopic confirmation of the brightest known gravitationally lensed Lyman break galaxy in the Epoch of Reionisation, A1703-zD1, through the detection of [C ii]158 μm at a redshift of z = 6.8269 ± 0.0004. This source was selected behind the strong lensing cluster Abell 1703, with an intrinsic $L_{\rm UV} \sim L^*_{\rm z=7}$ luminosity and a very blue Spitzer/IRAC [3.6]–[4.5] colour, implying high equivalent width line emission of [O iii]+Hβ. [C ii] is reliably detected at 6.1σ co-spatial with the rest-frame UV counterpart, showing similar spatial extent. Correcting for the lensing magnification, the [C ii] luminosity in A1703-zD1 is broadly consistent with the local $L_{\rm [C\, \small {II}]}$ – SFR relation. We find a clear velocity gradient of 103 ± 22 km $\rm s^{-1}$ across the source which possibly indicates rotation or an ongoing merger. We furthermore present spectral scans with no detected [C ii] above 4.6σ in two unlensed Lyman break galaxies in the EGS-CANDELS field at z ∼ 6.6–6.9. This is the first time that NOEMA has been successfully used to observe [C ii] in a ‘normal’ star-forming galaxy at z > 6, and our results demonstrate its capability to complement ALMA in confirming galaxies in the Epoch of Reionisation.