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 $\rm
<z>$ = 1.15 and further to $\rm <z>$ = 2.2 and 4.8, particularly amongst
galaxies above the SF galaxy main sequence (MS). The galaxies with highest SFRs
at $\rm <z>$ = 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 - 4$\times$ their
stellar masses. For the full sample of galaxies, we find a single, very simple
SF law: $\rm SFR \propto M_{\rm ISM}^{0.9}$, 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 $\sim$5 -
10$\times$ shorter gas depletion times ($\sim0.2$ Gyr) than galaxies at low
redshift. These {\bf 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.
