H i surface density contours (white) derived from Ott et al. (2012) maps are superimposed onto the stellar density maps obtained with the matched filter technique (greyscale). Contour levels are 3, 9, 27, 81, 240, and 720 × 1019 cm-2. The levels of grey correspond to density of 3, 5, 10, 20, 40, and 80σ above the background, from the lightest to the darkest shade of grey.
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