DM_IGM-z relation for localized FRBs. The red dots show localized FRBs. The blue line corresponds to the DM_IGM from the IllustrisTNG 300 simulation, and the blue shaded area is the 95% confidence region. The dashed orange and purple lines represent the models in Equation (5) with different parameters (H₀ and f_IGM). 〈DM_IGM〉 = DM_exc–〈DM_host〉, where 〈DM_host〉 is given by the IllustrisTNG simulation.