Using the EAGLE cosmological simulations, this study demonstrates that a physically motivated galaxy size metric (R1)—defined by the outer limit of in situ star formation—provides a high-precision link to dark matter halo properties. The simulated galaxies accurately reproduce the observed stellar mass-size relation with a remarkably low scatter of 0.06 dex. By establishing a tight correlation between R1 and the halo radius (R200), the authors show that halo size and mass can be inferred from a galaxy's physical "edge" with less than 50% uncertainty. This method is six times more accurate
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