Left column: gas density distribution (grey-plot) and velocity field in the rotating bar frame (vectors) in the galactic midplane. Middle column: column density of the gas (grey-scale) in the inner 6 kpc × 6 kpc region. Right column: polarized intensity (contours), polarization angles (dashes) and gas column density (grey-scale). Three different values of the sound speed are c_s = 5 km s^-1 (1st row), c_s = 7 km s^-1 (2nd row) and c_s = 10 km s^-1 (3rd row). All polarization maps have been normalized to their initial distribution and smoothed to the resolution 40''.