Comparison of two models from Allaert et al. (2015) without (top plot) and with arcs mimicking the effect of spiral arms (bottom plot), built on the basis of the kinematical information analyzed in their work. For both plots, the WISE W3 image is overplotted with the observed Hi contours (black) and the contours from a model (white). Contour levels are 1.49 × 10²¹, 4.475 × 10²¹ and 6.56 × 10²¹ atoms cm^-2.