2024ApJ...961...71F

We present results of [C II] 158 μm 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_158μm 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 × 10⁸L_⊙. 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.