Stars form from cold molecular interstellar gas. As this is relatively rare in the local Universe, galaxies like the Milky Way form only a few new stars per year. Typical massive galaxies in the distant Universe formed stars an order of magnitude more rapidly1,2. Unless star formation was significantly more efficient, this difference suggests that young galaxies were much more molecular-gas rich. Molecular gas observations in the distant Universe have so far largely been restricted to very luminous, rare objects, including mergers and quasars3,4,5, and accordingly we do not yet have a clear idea about the gas content of more normal (albeit massive) galaxies. Here we report the results of a survey of molecular gas in samples of typical massive-star-forming galaxies at mean redshifts <z> of about 1.2 and 2.3, when the Universe was respectively 40% and 24% of its current age. Our measurements reveal that distant star forming galaxies were indeed gas rich, and that the star formation efficiency is not strongly dependent on cosmic epoch. The average fraction of cold gas relative to total galaxy baryonic mass at z = 2.3 and z = 1.2 is respectively about 44% and 34%, three to ten times higher than in today’s massive spiral galaxies6. The slow decrease between z ≈ 2 and z ≈ 1 probably requires a mechanism of semi-continuous replenishment of fresh gas to the young galaxies.
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This work is based on observations carried out with the IRAM Plateau de Bure Interferometer. IRAM is supported by INSU/CNRS (France), MPG (Germany) and IGN (Spain). We thank B. Lazareff and the IRAM staff for their work in developing the new PdBI receiver systems, which made these technically difficult observations feasible. We are grateful to J. Blaizot, L.-M. Dansac, R. Davé, D. Kereŝ, P. Ocvirk, C. Pichon and R. Teyssier for communicating unpublished results of their simulations and for discussions. A.B. and T.N. thank the Cluster of Excellence "Origin and Structure of the Universe" for support. M.C.C. is a Spitzer Fellow; A.B. is a Max Planck Fellow.
Author Contributions All authors have contributed extensively to this manuscript.
The authors declare no competing financial interests.
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Tacconi, L., Genzel, R., Neri, R. et al. High molecular gas fractions in normal massive star-forming galaxies in the young Universe. Nature 463, 781–784 (2010). https://doi.org/10.1038/nature08773
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