Spiral Density Waves in M81. II. Hydrodynamic Simulations for Gas
  Response to Stellar Spiral Density Waves
  Gas response to the underlying stellar spirals is explored for M81 using
unmagnetized hydrodynamic simulations. Constrained within the uncertainty of
observations, 18 simulations are carried out to study the effects of
selfgravity and to cover the parameter space comprising three different sound
speeds and three different arm strengths. The results are confronted with those
data observed at wavelengths of 8 $\mu$m and 21 cm. In the outer disk, the
ring-like structure observed in 8 $\mu$m image is consistent with the response
of cold neutral medium with an effective sound speed 7 km s$^{-1}$, while for
the inner disk, the presence of spiral shocks can be understood as a result of
4:1 resonances associated with the warm neutral medium with an effective sound
speed 19 km s$^{-1}$. Simulations with single effective sound speed alone
cannot simultaneously explain the structures in the outer and inner disks. This
justifies the coexistence of cold and warm neutral media in M81. The
anomalously high streaming motions observed in the northeast arm and the
outward shifted turning points in the iso-velocity contours seen along the
southwest arm are interpreted as signatures of interactions with companion
galaxies. The level of simulated streaming motions narrows down the uncertainty
of observed arm strength toward larger amplitudes.
