H I gas distribution (green contours) overlayed on the continuum-subtracted Hα image of NGC 4523 derived using the VESTIGE narrow-band imaging data. Green contours are shown at column densities of N(HI) = 2ⁿ × 3.9 × 10¹⁹ cm⁻² with n = 0, 1, …5. The colour scale on the right gives the Hα surface brightness in units of erg s⁻¹ cm⁻² arcsec⁻². The H II regions located outside the stellar disc and analysed in Sect. 6 are identified with yellow circles. The yellow arrow indicates the direction of the cluster centre (M87), located at a projected distance of 0.83 Mpc.