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

¹ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany
² Cosmic Dawn Center (DAWN), Copenhagen N, Denmark
e-mail: francesco.valentino@eso.org
³ Department of Astronomy, The University of Texas at Austin, Austin, TX, USA
⁴ Niels Bohr Institute, University of Copenhagen, Jagtvej 128, 2200 Copenhagen N, Denmark
⁵ Institute of Astronomy, Graduate School of Science, The University of Tokyo, 2-21-1, Osawa, Mitaka, Tokyo, 181-0015, Japan
⁶ Research Center for Early Universe, Graduate School of Science, The University of Tokyo, 7-3-1, Hongo, Bunkyo-ku, Tokyo, 113-0033, Japan
⁷ Steward Observatory, University of Arizona, 933 N. Cherry Ave., Tucson, AZ, 85721, USA
⁸ Kapteyn Astronomical Institute, University of Groningen, 9700 AV Groningen, The Netherlands
⁹ Instituto de Astrofísica, Facultad de Física, Pontificia Universidad Católica de Chile, Campus San Joaquín, Av. Vicuña Mackenna 4860, Macul Santiago, 7820436, Chile
¹⁰ Centro de Astroingeniería, Facultad de Física, Pontificia Universidad Católica de Chile, Campus San Joaquín, Av. Vicuña Mackenna 4860, Macul Santiago, 7820436, Chile
¹¹ Millennium Institute of Astrophysics, Nuncio Monseñor Sótero Sanz 100, Of 104, Providencia, Santiago, Chile
¹² Space Science Institute, 4750 Walnut Street, Suite 205, Boulder, CO, 80301, USA
¹³ Dipartimento di Fisica, Sapienza, Università di Roma, Piazzale Aldo Moro 5, 00185 Roma, Italy
¹⁴ INFN, Sezione di Roma I, Piazzale Aldo Moro 2, 00185 Roma, Italy
¹⁵ INAF/Osservatorio Astronomico di Roma, Via di Frascati 33, 00078 Monte Porzio Catone, Italy
¹⁶ Departamento de Física Teórica y del Cosmos, Campus de Fuentenueva, Edificio Mecenas, Universidad de Granada, 18071 Granada, Spain
¹⁷ Instituto Carlos I de Física Teórica y Computacional, Facultad de Ciencias, 18071 Granada, Spain
¹⁸ DTU-Space, Technical University of Denmark, Elektrovej 327, 2800 Kgs. Lyngby, Denmark
¹⁹ Department of Astronomy, University of Geneva, Chemin Pegasi 51, 1290 Versoix, Switzerland
²⁰ Purple Mountain Observatory and Key Laboratory for Radio Astronomy, Chinese Academy of Sciences, Nanjing, PR China
²¹ School of Astronomy and Space Science, University of Science and Technology of China, Hefei, PR China
²² Space Telescope Science Institute, 3700 San Martin Dr., Baltimore, MD, 21218, USA
²³ Institute for Cosmic Ray Research, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba, 277-8582, Japan
²⁴ Kavli IPMU (WPI), The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba, 277-8583, Japan
²⁵ National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo, 181-8588, Japan
²⁶ Department of Physics & Astronomy, Johns Hopkins University, 3400 North Charles Street, Baltimore, MD, 21218, USA
²⁷ Max-Planck-Institut für Extraterrestrische Physik (MPE), Giessenbachstraße 1, 85748 Garching, Germany
²⁸ ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D), Australia
²⁹ International Centre for Radio Astronomy Research (ICRAR), University of Western Australia, Crawley, WA, 6009, Australia
³⁰ Institute for Advanced Research, Nagoya University, Furocho, Chikusa, Nagoya, 464-8602, Japan
³¹ Department of Physics, Graduate School of Science, Nagoya University, Furocho, Chikusa, Nagoya, 464-8602, Japan
³² Cahill Center for Astronomy and Astrophysics, California Institute of Technology, MS 249-17, Pasadena, CA, 91125, USA
³³ Academia Sinica Institute of Astronomy and Astrophysics (ASIAA), No. 1, Sec. 4, Roosevelt Rd., Taipei, 10617, Taiwan

Received: 2 October 2023
Accepted: 19 February 2024

Abstract

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.