1. Racah Institute of Physics, The Hebrew University, Jerusalem 91904, Israel
2. Harvard Smithsonian Center for Astrophysics, 60 Garden St, Cambridge, Massachusetts 02138, USA
3. Departement de Physique, ENS, 24 rue Lhomond, 75231 Paris cedex 05, France
4. Max Planck Institute for Astrophysics, Karl-Schwarzschild-Strasse 1, 85741 Garching, Germany
5. Institut d'Astrophysique de Paris and UPMC, 98bis Boulevard Arago, Paris 75014, France
6. CEA Saclay, DSM/IRFU, UMR AIM, Batiment 709, 91191 Gif-sur-Yvette cedex, France
7. Institute for Theoretical Physics, University of Zurich, CH-8057 Zurich, Switzerland

Correspondence to: A. Dekel¹ Correspondence and requests for materials should be addressed to A.D. (Email: dekel@phys.huji.ac.il).

Abstract

Massive galaxies in the young Universe, ten billion years ago, formed stars at surprising intensities^{1, 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 spheroids^{1, 2, 3, 4, 5}. Cosmological simulations⁶ and clustering theory^{6, 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 haloes^{8, 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 intensely^{2, 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 spheroid^{4, 10, 11}. This stream-driven scenario for the formation of discs and spheroids is an alternative to the merger picture.

1. Racah Institute of Physics, The Hebrew University, Jerusalem 91904, Israel
2. Harvard Smithsonian Center for Astrophysics, 60 Garden St, Cambridge, Massachusetts 02138, USA
3. Departement de Physique, ENS, 24 rue Lhomond, 75231 Paris cedex 05, France
4. Max Planck Institute for Astrophysics, Karl-Schwarzschild-Strasse 1, 85741 Garching, Germany
5. Institut d'Astrophysique de Paris and UPMC, 98bis Boulevard Arago, Paris 75014, France
6. CEA Saclay, DSM/IRFU, UMR AIM, Batiment 709, 91191 Gif-sur-Yvette cedex, France
7. Institute for Theoretical Physics, University of Zurich, CH-8057 Zurich, Switzerland

Correspondence to: A. Dekel¹ Correspondence and requests for materials should be addressed to A.D. (Email: dekel@phys.huji.ac.il).

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