The ALPINE-ALMA [C II] survey

Molecular gas budget in the early Universe as traced by [C II]

¹ Observatoire de Genève, Université de Genève, 51 Ch. des Maillettes, 1290 Versoix, Switzerland
e-mail: miroslava.dessauges@unige.ch
² Università di Bologna – Dipartimento di Fisica e Astronomia, Via Gobetti 93/2, 40129 Bologna, Italy
³ INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Gobetti 93/3, 40129 Bologna, Italy
⁴ Aix Marseille Université, CNRS, LAM (Laboratoire d’Astrophysique de Marseille) UMR 7326, 13388 Marseille, France
⁵ The Cosmic Dawn Center, University of Copenhagen, Vibenshuset, Lyngbyvej 2, 2100 Copenhagen, Denmark
⁶ Niels Bohr Institute, University of Copenhagen, Lyngbyvej 2, 2100 Copenhagen, Denmark
⁷ Kavli Institute for the Physics and Mathematics of the Universe, The University of Tokyo (Kavli IPMU, WPI), 277-8583 Kashiwa, Japan
⁸ Department of Astronomy, School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo, 113-0033 Japan
⁹ Cavendish Laboratory, University of Cambridge, 19 J. J. Thomson Ave., Cambridge, CB3 0HE UK
¹⁰ Kavli Institute for Cosmology, University of Cambridge, Madingley Road, Cambridge, CB3 0HA UK
¹¹ Leiden Observatory, Leiden University, PO Box 9500, 2300 RA Leiden, The Netherlands
¹² IPAC, M/C 314-6, California Institute of Technology, 1200 East California Boulevard, Pasadena, CA, 91125 USA
¹³ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
¹⁴ Dipartimento di Fisica e Astronomia, Università di Padova, Vicolo dell’Osservatorio, 3, 35122 Padova, Italy
¹⁵ INAF – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
¹⁶ The Caltech Optical Observatories, California Institute of Technology, Pasadena, CA, 91125 USA
¹⁷ Instituto de Investigación Multidisciplinar en Ciencia y Tecnología, Universidad de La Serena, Raúl Bitrán, 1305 La Serena, Chile
¹⁸ Departamento de Astronomía, Universidad de La Serena, Av. Juan Cisternas 1200 Norte, La Serena, Chile
¹⁹ Centro de Astronomía (CITEVA), Universidad de Antofagasta, Avenida Angamos 601, Antofagasta, Chile
²⁰ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
²¹ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD, 21218 USA
²² Instituto de Física y Astronomía, Universidad de Valparaíso, Avda. Gran Bretaña 1111, Valparaíso, Chile
²³ Department of Physics, University of California, Davis, One Shields Ave., Davis, CA, 95616 USA
²⁴ Department of Astronomy, University of Florida, 211 Bryant Space Sciences Center, Gainesville, FL, 32611 USA

Received: 22 April 2020
Accepted: 17 July 2020

Abstract

The molecular gas content of normal galaxies at z >  4 is poorly constrained because the commonly used molecular gas tracers become hard to detect at these high redshifts. We use the [C II] 158 μm luminosity, which was recently proposed as a molecular gas tracer, to estimate the molecular gas content in a large sample of main sequence star-forming galaxies at z = 4.4 − 5.9, with a median stellar mass of 10^9.7 M_⊙, drawn from the ALMA Large Program to INvestigate [C II] at Early times survey. The agreement between the molecular gas masses derived from [C II] luminosities, dynamical masses, and rest-frame 850 μm luminosities extrapolated from the rest-frame 158 μm continuum supports [C II] as a reliable tracer of molecular gas in our sample. We find a continuous decline of the molecular gas depletion timescale from z = 0 to z = 5.9, which reaches a mean value of (4.6 ± 0.8) × 10⁸ yr at z ∼ 5.5, only a factor of between two and three shorter than in present-day galaxies. This suggests a mild enhancement of the star formation efficiency toward high redshifts. Our estimates also show that the previously reported rise in the molecular gas fraction flattens off above z ∼ 3.7 to achieve a mean value of 63%±3% over z = 4.4 − 5.9. This redshift evolution of the gas fraction is in line with that of the specific star formation rate. We use multi-epoch abundance-matching to follow the gas fraction evolution across cosmic time of progenitors of z = 0 Milky Way-like galaxies in ∼10¹³ M_⊙ halos and of more massive z = 0 galaxies in ∼10¹⁴ M_⊙ halos. Interestingly, the former progenitors show a monotonic increase of the gas fraction with redshift, while the latter show a steep rise from z = 0 to z ∼ 2 followed by a constant gas fraction from z ∼ 2 to z = 5.9. We discuss three possible effects, namely outflows, a pause in gas supply, and over-efficient star formation, which may jointly contribute to the gas fraction plateau of the latter massive galaxies.