Constraints on the [CII] luminosity of a proto-globular cluster at z~6
  obtained with ALMA
  We report on ALMA observations of D1, a system at z~6.15 with stellar mass
M_* ~ 10^7 M_sun 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 dataset, 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 4-sigma tentative
detection of [CII] emission with intrinsic luminosity L_CII=(2.9 +/- 1.4) 10^6
L_sun, one of the lowest values ever detected at high redshift. This study
offers a first insight on previously uncharted regions of the L_CII-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 [CII] 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 3-sigma upper limit
values as low as a few 10^4 M_sun, consistent with the values measured in local
metal-poor galaxies.
