We have performed smoothed particle hydrodynamics ( SPH ) simulations to study the response of the central kiloparsec region of a gaseous disk to the imposition of nonaxisymmetric bar potentials .
The model galaxies are composed of the three axisymmetric components ( halo , disk , and bulge ) and a non-axisymmetric bar .
These components are assumed to be invariant in time in the frame corotating with the bar .
The potential of spherical \gamma -models of Dehnen is adopted for the bulge component whose density varies as r ^ { - \gamma } near the center and r ^ { -4 } at larger radii and hence , possesses a central density core for \gamma = 0 and cusps for \gamma > 0 .
Since the central mass concentration of the model galaxies increases with the cusp parameter \gamma , we have examined here the effect of the central mass concentration by varying the cusp parameter \gamma on the mechanism responsible for the formation of the symmetric two-armed nuclear spirals in barred galaxies .
Our simulations show that the symmetric two-armed nuclear spirals are formed by hydrodynamic spiral shocks driven by the gravitational torque of the bar for the models with \gamma = 0 and 0.5 .
On the other hand , the symmetric two-armed nuclear spirals in the models with \gamma = 1 and 1.5 are explained by gas density waves .
Thus , we conclude that the mechanism responsible for the formation of the symmetric two-armed nuclear spirals in barred galaxies changes from the hydrodynamic shocks to the gas density waves when the central mass concentration increases from \gamma = 0 to 1.5 .