HST/WFC3 F160W near-infrared image (top panel), [O II] emission line velocity map (middle left), CO line velocity map (middle right), [O II] emission line velocity dispersion map (bottom left) and CO line velocity dispersion map (bottom right) of A521 in the image plane. The PSF of 0.69″ of the kinematic maps is shown as a gray circle in the bottom left corner. Both velocity dispersion maps are corrected for the instrumental broadening, but uncorrected for beam-smearing. The critical line from the lensing model is shown in white in the top panel. The HST image and kinematic maps are at the same scale.