H I gas distribution (green contours) overlayed on the FUV ASTROSAT/UVIT image in the BaF2 filter (λ_c = 1541 Å; Δλ = 380 Å) (left panel) and on the GALEX image of NGC 4523 in the NUV filter (λ_c = 2316 Å Δλ = 1060 Å; right panel). Green contours are for H I gas column densities of N(HI) = 2ⁿ × 3.9 × 10¹⁹ cm⁻² with n = 0, 1, …5. The H II regions located outside the stellar disc and analysed in Sect. 6 are identified with blue circles. The blue arrow indicates the direction of the cluster centre (M87), located at 0.83 Mpc projected distance.