Massive galaxies in the young Universe, ten billion years ago, formed stars at surprising intensities1,2. Although this is commonly attributed to violent mergers, the properties of many of these galaxies are incompatible with such events, showing gas-rich, clumpy, extended rotating disks not dominated by spheroids1,2,3,4,5. Cosmological simulations6 and clustering theory6,7 are used to explore how these galaxies acquired their gas. Here we report that they are ‘stream-fed galaxies’, formed from steady, narrow, cold gas streams that penetrate the shock-heated media of massive dark matter haloes8,9. A comparison with the observed abundance of star-forming galaxies implies that most of the input gas must rapidly convert to stars. One-third of the stream mass is in gas clumps leading to mergers of mass ratio greater than 1:10, and the rest is in smoother flows. With a merger duty cycle of 0.1, three-quarters of the galaxies forming stars at a given rate are fed by smooth streams. The rarer, submillimetre galaxies that form stars even more intensely2,12,13 are largely merger-induced starbursts. Unlike destructive mergers, the streams are likely to keep the rotating disk configuration intact, although turbulent and broken into giant star-forming clumps that merge into a central spheroid4,10,11. This stream-driven scenario for the formation of discs and spheroids is an alternative to the merger picture.
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We acknowledge discussions with N. Bouche, S. M. Faber, R. Genzel, D. Koo, A. Kravtsov, A. Pope, J. R. Primack, J. Prochaska, A. Sternberg and J. Wall. This research was supported by the France–Israel Teamwork in Sciences, the German–Israel Science Foundation, the Israel Science Foundation, a NASA Theory Program at UCSC, and a Minerva fellowship (T.G.). We thank the Barcelona Centro Nacional de Supercomputación for computer resources and technical support. The simulation is part of the Horizon collaboration.
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Dekel, A., Birnboim, Y., Engel, G. et al. Cold streams in early massive hot haloes as the main mode of galaxy formation. Nature 457, 451–454 (2009). https://doi.org/10.1038/nature07648
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