Reaching for the Edge II: Stellar Halos out to Large Radii as a Tracer of Dark Matter Halo Mass

Abstract

The diffuse outskirts of brightest cluster galaxies (BCGs) encode valuable information about the assembly history and mass of their host dark matter halos. However, the low surface brightness of these stellar halos has historically made them difficult to observe. Recent deep imaging, particularly with Hyper Suprime-Cam (HSC), has shown that the stellar mass within relatively large projected annuli, such as within $50$ and $100$ kpc, is a promising proxy for halo mass. However, the optimal radial definition of this "outskirt mass" remains uncertain. We construct an HSC-like mock observing pipeline to measure the stellar mass density profiles of BCGs in the IllustrisTNG simulations. Our mock observations closely reproduce HSC profiles across six orders of magnitude in surface density. We then systematically measure stellar masses within different annuli and how tightly they are connected to halo mass. We find that stellar masses measured within simple apertures exhibit considerably more scatter in the stellar mass-halo mass relation than those measured within projected ellipsoidal annuli. We identify an optimal range of definitions, with inner radii between $\sim 70$-$200$ kpc and outer radii between $\sim 125$-$500$ kpc. We also introduce two halo-mass-dependent Sérsic models for the average stellar halo profiles. We present a Sérsic-based fitting function that describes the profiles as a function of the halo mass, $M_{\rm vir}$, with a median error of $54\%$. Adding the central stellar mass of the BCG as a second parameter slightly improves the accuracy to a median error of $39\%$. Together, these results provide fitting functions for BCG stellar halos that can be applied to future wide-field surveys to infer halo masses from deep imaging data.