Archival HST/ACS image (color and contours, shown in a square root scale) of Mrk 231. The observation was taken with the optical F435W filter and it shows the central 16″ × 16″, which corresponds to a region of ≃13.4 kpc × 13.4 kpc in size. For reference, the overlaid dashed square represents the approximate FoV and orientation of the MRS ch3 observations. We mark the positions of several expanding shells identified by Lípari et al. (2005) and discussed in the text.