  We use a series of idealized, numerical SPH simulations to study the
formation and evolution of galactic, gas-rich disks forming from gas infall
within dark matter halos. The temperature and density structure of the gas is
varied in order to differentiate between (i) simultaneous gas infall at a large
range of radii and (ii) the inside-out build-up of a disk. In all cases, the
disks go through phases of ring formation, gravitational instability and
break-up into massive clumps. Ring formation can be enhanced by a focal point
effect. The position of the ring is determined by the angular momentum
distribution of the material it forms from. We study the ring and clump
morphologies, the characteristic properties of the resulting velocity
dispersion field and the effect of star formation. In the early phases, gas
accretion leads to a high vertical velocity dispersion. We find that the disk
fragmentation by gravitational instability and the subsequent clump-clump
interactions drive high velocity dispersions mainly in the plane of the disk
while at the same time the vertical velocity dispersion dissipates. The result
is a strong variation of the line-of-sight velocity dispersion with inclination
angle. For a face-on view, clumps appear as minima in the (vertical)
dispersion, whereas for a more edge-on view, they tend to correspond to maxima.
There exists observational evidence of a systematic variation of the velocity
dispersion with inclination angle in high-redshift disks, which could be partly
explained by our simulation results. Additional energetic sources to drive
velocity dispersion that are not included in our models are also expected to
contribute to the observational results.
