The ALPINE-ALMA [C II] survey. Molecular gas budget in the early Universe as traced by [C II]

November 2020 • 2020A&A...643A...5D

Authors • Dessauges-Zavadsky, M. • Ginolfi, M. • Pozzi, F. • Béthermin, M. • Le Fèvre, O. • Fujimoto, S. • Silverman, J. D. • Jones, G. C. • Vallini, L. • Schaerer, D. • Faisst, A. L. • Khusanova, Y. • Fudamoto, Y. • Cassata, P. • Loiacono, F. • Capak, P. L. • Yan, L. • Amorin, R. • Bardelli, S. • Boquien, M. • Cimatti, A. • Gruppioni, C. • Hathi, N. P. • Ibar, E. • Koekemoer, A. M. • Lemaux, B. C. • Narayanan, D. • Oesch, P. A. • Rodighiero, G. • Romano, M. • Talia, M. • Toft, S. • Vergani, D. • Zamorani, G. • Zucca, E.

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 109.7 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.


IPAC Authors


Andreas Faisst

Assistant Scientist