The Outskirt Stellar Mass of Low-redshift Massive Galaxies Is an Excellent Halo Mass Proxy in Illustris/IllustrisTNG Simulations

Recent observations suggest that the extended stellar halos of low-redshift massive galaxies are tightly connected to the assembly of their dark matter halos. In this paper, we use the Illustris, IllustrisTNG100, and IllustrisTNG300 simulations to compare how different stellar aperture masses trace halo mass. For massive central galaxies (M⋆ ≥ 1011.2 M⊙), we find that a 2D outskirt stellar mass measured between 50 and 100 kpc (M⋆,[50,100]) consistently outperforms other aperture-based stellar masses. We further show that M⋆,[50,100] correlates better with halo mass than the total mass of accreted stars (the ex situ mass), which suggests that not all accreted stars connect to halo assembly equally. While the galaxy formation recipes differ between Illustris and IllustrisTNG100, the two simulations yield consistent ex situ outskirt fractions for massive galaxies (∼70% in M⋆,[50,100]). These results demonstrate the potential of using the outskirt stellar mass to deepen our understanding of the galaxy–halo connection in massive dark matter halos and to trace dark matter halos better.