A NOEMA pilot survey of Cii line emission in luminous galaxies at z~6

We present Northern Extended Millimeter Array (NOEMA) observations of four luminous galaxies at (z ∼ 6), when the Universe was less than (∼ 1) Gyr old. These galaxies are detected in the ̧ii 158, μm fine-structure line, and three also exhibit underlying dust continuum emission. The ̧ii line velocities are offset by a few hundred kilometers per second relative to the Lyα emission, possibly indicating the presence of outflows or a large reservoir of surrounding atomic gas. From the combination of dust continuum and rest-frame UV emission, we estimate that approximately 50% of the star formation is obscured by dust, consistent with values found in other massive star-forming galaxies at similar redshifts. The ̧ii-to-far-infrared (FIR) luminosity ratios of the continuum-detected galaxies are elevated compared to nearby galaxies with similar FIR luminosities. In one system, J163026+4315, the ̧ii emission is spatially resolved, allowing us to measure the source size and FIR surface brightness FIR). We find that the ̧ii/FIR as a function of Σ_ FIR in this source follows the trends observed in both local and high-redshift galaxy populations. A kinematic analysis of the resolved ̧ii emission provides tentative evidence for a rotating disk, although higher-angular resolution data are needed to confirm this. These results demonstrate the power of NOEMA to probe the physical conditions and structure of the interstellar medium in galaxies in the first billion years of cosmic history.