  The massive galaxies in the young universe, ten billion years ago, formed
stars at surprising intensities. 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 (Genzel et al. 2006, 2008). Cosmological simulations and clustering
theory 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
(Dekel & Birnboim 2006; Keres et al. 2005). 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 duy 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 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 (Noguchi 1999; Genzel et al. 2008,
Elmegreen, Bournaud & Elmegreen 2008, Dekel, Sari & Ceverino 2009). This
stream-driven scenario for the formation of disks and spheroids is an
alternative to the merger picture.
