Low-resolution (27″ × 39″; left) and high-resolution (12″ × 21″; right) H I gas distribution of NGC 4523. Green contours are at column densities of N(HI) = 2ⁿ × 3.9 × 10¹⁹ cm⁻² with n = 0, 1, …5. The black contour shows the i-band isophote at 24.5 mag arcsec⁻². The open dot shows the kinematic centre of the galaxy, the ellipse indicates the size of the beam, the arrow the direction of the cluster centre (M87), located at 0.83 Mpc projected distance.