HST [OIII]λ5007 Å image and Toomre Q-parameter map of Mrk 509. Right panel: HST image in the FQ508N filter, probing rest-frame [OIII]λ5007 Å emission (greyscale). The red contours represent CO(2−1) emission (Fig. 1centre panel, contours are drawn at (1, 2, 3, 4, 5, 7, 10, 15, 20, 25, 30, 37) × σ). The blue contours represent CO(2−1) emission in region A and B (Fig. 6 top panel, contours are drawn at (3, 4, 5) × σ). Image from the Mikulski Archive for Space Telescopes (MAST), HST Proposal 12212. Right panel: Toomre Q-parameter map for the cold molecular gas component. The black contours represent the moment 0 CO emission at (2, 5, 15, 30, 45) × σ. The white cross gives the continuum peak position. The white dashed ellipses indicate projected annuli with radius R = 0.3, 0.8, 1.4, 2.2 arcsec (see Table 3).