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 Reionization (EoR), A1703-zD1, through the detection of [C II] 158 mu m at a redshift of z = 6.8269 +/- 0.0004. This source was selected behind the strong lensing cluster Abell 1703, with an intrinsic luminosity and a very blue Spitzer/Infrared Array Camera (IRAC) [3.6]-[4.5] colour, implying high equivalent width line emission of [O III] + H beta. [C II] is reliably detected at 6.1 sigma cospatial with the rest-frame ultraviolet (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-[C II]-star formation rate (SFR) relation. We find a clear velocity gradient of 103 +/- 22 km s(-1) across the source that possibly indicates rotation or an ongoing merger. We furthermore present spectral scans with no detected [C II] above 4.6 sigma in two unlensed Lyman-break galaxies in the Extended Groth Strip (EGS)-Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS) field at z similar to 6.6-6.9. This is the first time that the Northern Extended Millimeter Array (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 the Atacama Large Millimeter/submillimeter Array (ALMA) in confirming galaxies in the EoR.

Automated summary

This study confirms the brightest known gravitationally lensed Lyman-break galaxy in the Epoch of Reionization (EoR), A1703-zD1, through spectroscopic detection of [C II] 158 mu m at a redshift of z = 6.8269. The [C II] luminosity in this galaxy is broadly consistent with the local star formation rate, and a velocity gradient within the source possibly indicates rotation or an ongoing merger. Additionally, the study demonstrates the capability of the Northern Extended Millimeter Array (NOEMA) to complement the Atacama Large Millimeter/submillimeter Array (ALMA) in confirming galaxies in the EoR.