2021MNRAS.500.3083C

We report on ALMA observations of D1, a system at z ∼ 6.15 with stellar mass M_* ∼10⁷M_☉ containing globular cluster (GC) precursors, strongly magnified by the galaxy cluster MACS J0416.1-2403. Since the discovery of GC progenitors at high redshift, ours is the first attempt to probe directly the physical properties of their neutral gas through infrared observations. A careful analysis of our data set, performed with a suitable procedure designed to identify faint narrow lines and which can test various possible values for the unknown linewidth value, allowed us to identify a 4σ tentative detection of [C II] emission with intrinsic luminosity L_[C II]_=(2.9 ±1.4)10⁶ L_☉, one of the lowest values ever detected at high redshift. This study offers a first insight on previously uncharted regions of the L_[C II]_ - SFR relation. Despite large uncertainties affecting our measure of the star formation rate, if taken at face value our estimate lies more than ∼1 dex below the values observed in local and high redshift systems. Our weak detection indicates a deficiency of [C II] emission, possibly ascribed to various explanations, such as a low-density gas and/or a strong radiation field caused by intense stellar feedback, and a low metal content. From the non-detection in the continuum, we derive constraints on the dust mass, with 3σ upper limit values as low as ∼ a few 10⁴ M_☉, consistent with the values measured in local metal-poor galaxies.