Fig. 4.
Hα map in polar projection, assuming a circular thin disk and the geometric parameters in Table 1. Linear structures in this ln(r) − ø parameter space correspond to logarithmic spirals. Four such structures are identified by eye and marked as S1, S2, S3, and S4. The resulting spiral arm mask in the plane of the sky is shown in Fig. 5. Details are described in Sect. 3.2.
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