Presentation #238.01 in the session Henry Norris Russell Lecture.
At present epochs stars dominate the baryonic mass of galaxies; however, at early times the gas which fuels both star formation and AGN accretion must have been dominant. We have used ALMA archive data in the COSMOS 2-deg2 field to measure the gas masses from the long wavelength dust continuum for unprecedented samples of 700 and 10,000 galaxies at redshift 0.1 to 6. Based on a sample of 128 calibration galaxies, these mass estimates have much higher accuracy than those from high J CO lines. We find measured gas masses of 1-300 billion solar masses — the latter being 100 times that of the Milky Way. Earlier than cosmic age 5 Gyr (z = 1.2), the interstellar gas was the dominant baryonic mass component, peaking at z = 1.9, and likely the main driver of early galaxy evolution. The SFR and AGN peak at z = 2 (cosmic age 3.3 Gyr) coincides with the peak in the gas contents. The correlations of gas masses and rates of star formation suggest two modes of star formation. The factor ~20 increase in star formation from the present back to 3 Gyr after the Big Bang, is largely due to the more abundant gas contents of early galaxies. On the other hand, starbursting activity in early galaxies exhibits higher star formation efficiencies and is likely due dynamical compression of the gas associated with galaxy interactions and merging.