This figure shows, in red, the F150W2 surface brightness profile extracted from the central galaxy in A402-BCG along a wedge as shown in the inset. For comparison, we show the galaxies with the largest cores measured to date, Holm 15A (K. Mehrgan et al. 2019) and IC 1101 (B. T. Dullo et al. 2017), and a sample of cored galaxies from B. T. Dullo (2019). All profiles have been corrected for cosmological dimming and k-corrected to the rest-frame V band assuming an old stellar population. The vertical red dotted line shows the location of the break radius measured from the two-dimensional fit described in Appendix A.1. This figure highlights the similarities between IC 1101 and A402-BCG, and suggests that A402-BCG likely harbors an ultramassive black hole.