Abstract.A new N-body and hydrodynamical code, called RAMSES, is presented. It has been designed to study structure formation in the universe with high spatial resolution. The code is based on Adaptive Mesh Refinement (AMR) technique, with a tree-based data structure allowing recursive grid refinements on a cell-by-cell basis. The N-body solver is very similar to the one developed for the ART code (Kravtsov et al. 1997), with minor differences in the exact implementation. The hydrodynamical solver is based on a second-order Godunov method, a modern shock-capturing scheme known to compute accurately the thermal history of the fluid component. The accuracy of the code is carefully estimated using various test cases, from pure gas dynamical tests to cosmological ones. The specific refinement strategy used in cosmological simulations is described, and potential spurious effects associated with shock waves propagation in the resulting AMR grid are discussed and found to be negligible. Results obtained in a large N-body and hydrodynamical simulation of structure formation in a low density ΛCDM universe are reported, with 256 3 particles and 4.1 × 10 7 cells in the AMR grid, reaching a formal resolution of 8192 3 . A convergence analysis of different quantities, such as dark matter density power spectrum, gas pressure power spectrum and individual haloe temperature profiles, shows that numerical results are converging down to the actual resolution limit of the code, and are well reproduced by recent analytical predictions in the framework of the halo model.

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. 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. 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 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. This stream-driven scenario for the formation of discs and spheroids is an alternative to the merger picture.