  The formation and evolution of galactic disks are complex phenomena, where
gas and star dynamics are coupled through star formation and the related
feedback. The physical processes are so numerous and intricate that numerical
models focus, in general, on one or a few of them only. We propose here a
numerical model with particular attention to the multiphase nature of the
interstellar medium; we consider a warm gas phase (> 10^4 K), treated as a
continuous fluid by an SPH algorithm, and a cold gas phase (down to 10K),
fragmented in clouds, treated by a low-dissipation sticky particles component.
The two gas phases do not have the same dynamics, nor the same spatial
distribution. In addition to gravity, they are coupled through mass exchanges
due to heating/cooling processes, and supernovae feedback. Stars form out of
the cold phase, and re-inject mass to the warm phase through SN explosions and
stellar winds. The baryons are embedded in a live cold dark matter component.
  Baryonic disks, initially composed of pure gas, encounter violent
instabilities, and a rapid phase of star formation, that slows down
exponentially. Stars form in big clumps, that accumulate in the center to build
a bulge. Exponential metallicity gradients are obtained. External infall of gas
should be included to maintain a star formation rate in the disk comparable to
what is observed in present disk galaxies.
