The cold interstellar medium of a normal sub-L⋆ galaxy at the end of reionization

We present the results of a ∼60-h multiband observational campaign with the Atacama Large Millimeter Array targeting a spectroscopically confirmed and lensed sub-L^⋆ galaxy at z = 6.07, first identified during the ALMA Lensing Cluster Survey (ALCS). We sampled the dust continuum emission from rest frame 90-370 μm at six different frequencies and set constraining upper limits on the molecular gas line emission and content by targeting the CO (7 − 6) and [C I](³P₂−³P₁) transitions in two lensed images with μ ≳ 20. Complementing these submillimeter observations with deep optical and near-IR photometry and spectroscopy with JWST, we find this galaxy to form stars at a rate of SFR ∼ 7 M_⊙ yr⁻¹, ∼50 − 70% of which is obscured by dust. This is consistent with what one would predict for a M_⋆ ∼ 7.5 × 10⁸ M_⊙ object by extrapolating the relation between the fraction of the obscured star formation rate and stellar mass at z < 2.5 and with observations of IR-detected objects at 5 < z < 7. The light-weighted dust temperature of T_dust ∼ 50 K is similar to that of more massive galaxies at similar redshifts, although with large uncertainties and with possible negative gradients. We measure a dust mass of M_dust ∼ 1.5 × 10⁶ M_⊙ and, by combining [C I], [C II], and a dynamical estimate, a gas mass of M_gas ∼ 2 × 10⁹ M_⊙. Their ratio (δ_DGR) is in good agreement with predictions from models and empirical relations in the literature. The dust-to-stellar mass fraction of f_dust ∼ 0.002 and the young stellar age (100 − 200 Myr) are consistent with efficient dust production via supernovae, as predicted by existing models and simulations of dust evolution. Also, the expected number density of galaxies with M_dust ∼ 10⁶ M_⊙ at z = 6 from a subset of these models is in agreement with the observational estimate that we set from the parent ALCS survey. The combination of gravitational lensing and deep multiwavelength observations allowed us to probe luminosity and mass regimes up to two orders of magnitude lower than what has been explored so far for field galaxies at similar redshifts. Our results serve as a benchmark for future observational endeavors of the high-redshift and faint sub-L^⋆ galaxy population that might have driven the reionization of the Universe.