Fig. 2.
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Annotated Hα surface brightness contours (contour levels SBHα = {1.5, 2.5, 5, 12.5, 100, 700}×10−18 erg s−1 cm−2 arcsec−2). In the background the Hα narrowband is shown with a log-stretch to 5 × 10−16 erg s−1 cm−2. The thin-dotted line surrounding the filaments to the north-west demarcates the aperture that is used to estimate the total Hα flux from that structure (Sect. 4).
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