The ALPINE-ALMA [C II] survey

Abstract

The molecular gas content of normal galaxies atz >  4 is poorly constrained because the commonly used molecular gas tracers become hard to detect at these high redshifts. We use the [C II] 158μm 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 atz = 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 850μm luminosities extrapolated from the rest-frame 158μm 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 fromz = 0 toz = 5.9, which reaches a mean value of (4.6 ± 0.8) × 108yr atz ∼ 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 abovez ∼ 3.7 to achieve a mean value of 63%±3% overz = 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 ofz = 0 Milky Way-like galaxies in ∼1013 M⊙halos and of more massivez = 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 fromz = 0 toz ∼ 2 followed by a constant gas fraction fromz ∼ 2 toz = 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.