Reconciling the Fundamental Plane of Early-Type Galaxies with hydrodynamical simulations

The Fundamental Plane (FP) of early-type galaxies (ETGs) encapsulates a tight correlation among structural and dynamical properties and serves as a benchmark for galaxy formation models. Yet, cosmological hydrodynamical simulations have historically failed to reproduce its observed tilt, with discrepancies attributed to flawed feedback physics or insufficient resolution. Here we show that applying observationally motivated measurements – including Sérsic derived photometry and dynamically inferred velocity dispersions – resolves this discrepancy within the IllustrisTNG-100 simulation. We further demonstrate that by mimicking a non-universal, mass-dependent stellar initial mass functon(IMF) in the dynamically corrected dispersion, improves consistency with observations across both direct and orthogonal FP fits. Our results suggest, for the first time, that mismatches between simulations and observations may stem not from inaccurate feedback but from how galaxy properties are measured. This work identifies observational realism and IMF variation as crucial ingredients for interpreting structural scaling laws in hydrodynamical simulations, offering a pathway to more predictive galaxy formation models without re-tuning feedback prescriptions.