Upper panels: distributions of HII regions (left) and all three kinds of spiral tracers (right) with the flat rotation curve with newly observed R₀ = 8.3 kpc and Θ₀ = 239 km s^-1 (Brunthaler et al. 2011). The symbols are the same as those in Fig. 2. The best-fitted four-arm models are also shown. Lower panels: best-fitted PL models for these two distributions are presented.