Gas surface density distributions in the fiducial model at several time instances focusing on the inward migration of IF1. The time is counted from t₀ = 153.5 kyr. The position of IF1 is marked with the yellow arrows, while the positions of OF1, OF2, and OF3 are marked with the red arrows. The yellow contour lines outline disk regions with the Toomre Q-parameter smaller than unity, while the black contour lines show the disk regions satisfying both the Toomre and Gammie criteria for disk fragmentation. The disk rotates counter clockwise.