Spectroscopic confirmation of a gravitationally lensed Lyman break galaxy at z$_{[CII]}$ = 6.827 using NOEMA

TL;DR

This paper reports the spectroscopic confirmation of a highly magnified Lyman break galaxy at z=6.827 using NOEMA, demonstrating its capability to observe [C II] in early galaxies and revealing potential rotation or merger activity.

Contribution

First successful NOEMA observation of [C II] in a normal star-forming galaxy at z>6, confirming a gravitationally lensed galaxy during the Epoch of Reionisation.

Findings

[C II] detected at 6.1σ with spatial coincidence to UV counterpart

[C II] luminosity consistent with local L$_{[CII]}$-SFR relation

Velocity gradient suggests rotation or merger activity

Abstract

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] at a redshift of z = 6.8269 +/- 0.0004. This source was selected behind the strong lensing cluster Abell 1703, with an intrinsic L$_{UV}$ ~ L$^*$$_{z=7}$ luminosity and a very blue Spitzer/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$\sigma$ 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$_{[CII]}$ - SFR relation. We find a clear velocity gradient of 103 +/- 22 km/s across the source which 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 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.