  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 radiiand 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.
