Constraints on the [C ii] luminosity of a proto-globular cluster at z ∼ 6 obtained with ALMA

Abstract

ABSTRACT We report on ALMA observations of D1, a system at z ∼ 6.15 with stellar mass $M_{*} \sim 10^7 \, \mathrm{M}_{\odot }$ 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 $L_{\rm [C\, \rm {\small II}]}=(2.9 \pm 1.4)~10^6 \, {\it L}_{\odot }$, one of the lowest values ever detected at high redshift. This study offers a first insight on previously uncharted regions of the $L_{\rm [C\, \rm {\small II}]}{-}{\rm SFR}$ 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.