  We analyze the spiral structure that results in a numerical simulation of a
galactic disk with stellar and gaseous components evolving in a potential that
includes an axisymmetric halo and bulge. We perform a second simulation without
the gas component to observe how it affects the spiral structure in the disk.
To quantify this, we use a Fourier analysis and obtain values for the pitch
angle and the velocity of the self-excited spiral pattern of the disk. The
results show a tighter spiral in the simulation with gaseous component. The
spiral structure is consistent with a superposition of waves, each with a
constant pattern velocity in given radial ranges.
