Upper panel. Distribution of the ratio of the X-ray extent of the detection to the total extent of X-ray emission, for the fossil group candidates (ΔM_1,2 ≥ 1.7) (dotted blue histogram) and non-fossils (ΔM_1,2 ≤ 0.5) (solid red histogram) in our X-ray group catalog. The fraction of fossil groups is skewed toward the lower values of compared to the non-fossil groups. Middle panel. The distribution for X-ray groups in different subsamples. Lower panel. The redshift evolution of the extent of X-ray detection for galaxy groups/clusters. A redshift evolution in the ratio by 0.25 in amplitude is seen for all groups.