ISM Masses and Star Formation at z = 1 to 6 ALMA Observations of Dust Continuum in 180 Galaxies in COSMOS

ALMA Cycle 2 observations of the long wavelength dust emission in 180 star-forming (SF) galaxies are used to investigate the evolution of ISM masses at z = 1 to 6.4. The ISM masses exhibit strong increases from z = 0 to = 1.15 and further to = 2.2 and 4.8, particularly amongst galaxies above the SF galaxy main sequence (MS). The galaxies with highest SFRs at = 2.2 and 4.8 have gas masses 100 times that of the Milky Way and gas mass fractions reaching 50 to 80%, i.e. gas masses 1 – 4x their stellar masses. For the full sample of galaxies, we find a single, very simple SF law: SFR ∝ M^(0.9)_(ISM), i.e. a 'linear' dependence on the ISM mass { on and above the MS. Thus, the galaxies above the MS are converting their larger ISM masses into stars on a timescale similar to those on the MS. At z > 1, the entire population of star-forming galaxies has ~5 – 10x shorter gas depletion times (~ 0.2 Gyr) than galaxies at low redshift. These shorter depletion times are due to a different, dominant mode of SF in the early universe - dynamically driven by compressive, high dispersion gas motions and/or galaxy interactions. The dispersive gas motions are a natural consequence of the extraordinarily high gas accretion rates which must occur to maintain the prodigious SF.