Difference between the redshift distribution functions of galaxies located within 0.5′ of the main ultra-massive lens galaxy at z = 0.581, and over the rest of the 2.5′ × 2.5′ field, after normalizing to the same area (orange curve). The black histogram is obtained from the peak of the redshift distribution of each source. A total of 14 galaxies within 0.5′ of the main deflector have redshifts consistent, at the 2σ confidence level, with z = 0.581, and 39 over the rest of our analyzed field.