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

TL;DR

This study shows that measuring stellar mass in the outer regions of massive galaxies in simulations provides a highly accurate proxy for their dark matter halo mass, improving our understanding of galaxy-halo connections.

Contribution

It introduces the outskirt stellar mass between 50-100 kpc as a superior halo mass proxy in simulations, outperforming other stellar mass measures.

Findings

Outskirt stellar mass correlates strongly with halo mass.

Outskirt stellar mass outperforms total accreted stars as a halo proxy.

Consistent results across different galaxy formation simulations.

Abstract

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 ($M_\star\geq 10^{11.2}M_\odot$), we find that a 2D outskirt stellar mass measured between 50 to 100 kpc ($M_{\star,[50,100]}$) consistently outperforms other aperture-based stellar masses. We further show that $M_{\star,[50,100]}$ correlates better with halo mass than the total amount of accreted stars (the ex situ mass), which suggests that not all accreted stars connect to halo assembly equally. While the galaxy formation recipes are different between Illustris and IllustrisTNG100, the two simulations yield consistent ex situ outskirt fractions for massive galaxies (about 70% in $M_{\star,[50,100]}$). These results demonstrate the potential of using the outskirt stellar mass to deepen our understanding of galaxy-halo connection in massive dark matter halos and trace dark matter halos better.