Significant structures and velocities of RCW 48. Upper panels: SHS Hα (A), radio continuum SUMSS (843 MHz) (B), Spitzer 8 μm (C), and WISE 22 μm (D) images of RCW 48. Lower panel: Velocity (in km s⁻¹) of the different components (sorted by decreasing peak intensities) of the Hα profiles averaged over the areas shown in the Hα image. The blue arrow is the tangential velocity vector of the star WR18. The red and blue circles delineate the small and large shells discussed in the text, respectively.