  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. 97), 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 to 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 LCDM universe are finally 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 haloes 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.
