Inferring the Total Mass of Galaxy Halos Using Photometry

Galaxy size, as defined as the radius at which the stellar surface density reaches 1~M_\,pc⁻² (R₁), has been shown to correlate more tightly with stellar mass than the effective radius. Cosmological simulations further predict a tight relation between R₁ and dark matter halo mass (M₂₀₀), but this prediction has not yet been tested observationally. We present the first observational test of the predicted R₁--M₂₀₀ relation and assess whether galaxy size, defined through R₁, can serve as a robust estimator of dark matter halo mass. We analyze galaxies from the SPARC database, combining high-quality rotation curves to infer dark matter halo masses with stellar surface density profiles derived from photometric data to measure R₁. We then compare the observed relation with the predictions from state-of-the-art cosmological simulations. We find good agreement between observations and simulations: galaxies follow the same tight R₁--M₂₀₀ relation predicted by numerical models across the explored mass range, with low intrinsic scatter. We provide the first observational validation of the R₁--M₂₀₀ relation, demonstrating that R₁ is a simple, physically motivated, and robust estimator of dark matter halo mass. Because R₁ is determined entirely from photometric observations, it provides a tool for inferring halo masses from imaging data alone, with applications to studies of galaxy formation and evolution.