Upper panels: best-fitted three-arm model (left) and four-arm model (right) for the distribution of HII regions. Lower panels: best-fitted three-arm model (left) and four-arm model (right) for the distribution of three kinds of spiral tracers. The symbols are the same as those in Fig. 2. The open red ellipse indicates the Galactic bar (Churchwell et al. 2009), and the laurel-gray ellipse shows the best-fitted Near 3 kpc Arm and Far 3 kpc Arm (Green et al. 2011). The dashed-lines indicate the observed tangential directions (see Table 2).
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