Fourier transform of the projected density in the spiral arms for the β = ∞ and β = 50 runs. The two solid lines show the Fourier transforms at t = 0.75, 0.90 T_rot, analogous to the times shown in Fig. 1. The wave number k here is given in units of 1/L_spiral, i.e., k = 1 corresponds to the full length of the spiral arm under consideration. We can see that the β = 50 run has more power at lower k (larger scales) than the hydro run, as we visually saw in Fig. 2.