The Extended [C ii] under Construction? Observation of the Brightest High-z Lensed Star-forming Galaxy at z = 6.2

We present results of [C ii ] 158 μ m emission line observations, and report the spectroscopic redshift confirmation of a strongly lensed ( μ ∼ 20) star-forming galaxy, MACS0308-zD1 at z = 6.2078 ± 0.0002. The [C ii ] emission line is detected with a signal-to-noise ratio >6 within the rest-frame UV-bright clump of the lensed galaxy (zD1.1) and exhibits multiple velocity components; the narrow [C ii ] has a velocity full width half maximum (FWHM) of 110 ± 20 km s ^−1 , while broader [C ii ] is seen with an FWHM of 230 ± 50 km s ^−1 . The broader [C ii ] component is blueshifted (−80 ± 20 km s ^−1 ) with respect to the narrow [C ii ] component, and has a morphology that extends beyond the UV-bright clump. We find that, while the narrow [C ii ] emission is most likely associated with zD1.1, the broader component is possibly associated with a physically distinct gas component from zD1.1 (e.g., outflowing or inflowing gas). Based on the nondetection of λ _158 _μ _m dust continuum, we find that MACS0308-zD1's star formation activity occurs in a dust-free environment indicated by a strong upper limit of infrared luminosity ≲9 × 10 ^8 L _⊙ . Targeting this strongly lensed faint galaxy for follow-up Atacama Large Millimeter/submillimeter Array and JWST observations will be crucial to characterize the details of typical galaxy growth in the early Universe.