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

TL;DR

This study demonstrates that the diffuse stellar halos of BCGs contain valuable information about their host halo mass. By building a realistic HSC-like mock observing pipeline on IllustrisTNG simulations, the authors measure 2D stellar mass density profiles out to 500 kpc and systematically test how different radial mass definitions trace $M_{ m vir}$. They find that stellar mass defined within intermediate ellipsoidal annuli—with inner radii around $70$–$200$ kpc and outer radii around $125$–$500$ kpc—maps to halo mass with reduced scatter compared to aperture masses, and they introduce two halo-mass dependent Sérsic meta-models that accurately describe the average halo profiles. These results enable forward-modeling of halo masses from deep imaging and will be valuable for interpreting next-generation wide-field surveys.

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.

Figures (10)

• Figure 1: Example of our mock observing routine applied to a galaxy from TNG100. a) The projected stellar mass distribution, integrating all stellar particles over a $20$ Mpc line-of-sight depth centered on the galaxy's center of mass. b) The initial isophotal model of the central galaxy. c) The map of the residuals after subtracting the initial model from the total stellar mass distribution, with detected satellite galaxies highlighted in a lighter color. d) The secondary isophotal model following satellite subtraction. e) The final isophotal model of the central galaxy with constant ellipticity.
• Figure 2: The Stellar Mass Halo Mass Relation (SHMR) for central galaxies in TNG50, TNG100, and TNG300 at z=0.4, showing the effect of simulation resolution on SHMRs. On the left are the SHMRs for central galaxies over a large mass range, while on the right are relationships for the sample of BCGs used in this study. Left: The median halo mass within bins of $0.15$ dex in stellar mass. Here the stellar mass is defined as the total 3D stellar mass bound to the central galaxy by SUBFIND. The shaded regions represent the $16^{\rm th}$ and $84^{\rm th}$ percentile scatter. the dots denote objects in bins too small to calculate a reliable median. The rescaled TNG300 stellar masses are shown by the dashed orange line. Right: The median halo mass as a function of stellar mass for the sample of BCGs used in this study. Here stellar mass is defined as the 2D stellar mass from mock-observed maps within a $100$ kpc semi-major axis. We compare the simulations to the observed HSC sample, which is plotted in black. At low masses, the TNG50 sample is the best match to the HSC sample, while at high masses, the rescaled TNG300 sample becomes a better match to the observations.
• Figure 3: Top: Median stellar surface mass density profiles for halos in the mass range $13.2 \leq \log_{10}(M_{\rm vir}/M_\odot) \leq 13.5$, shown for TNG50 (left), TNG100 (middle), and TNG300 (right). Computed using different particles and geometries. Middle: Comparison of FOF (light blue) and all particle (pink) profiles calculated in circular annuli. Within the FOF sample, the central galaxy (dashed) dominated over satellites (dotted) out to ($\approx 0.01 R_{500}$). Beyond $\sim0.1,R_{500}$, the all-particle profiles (solid pink) exceed the FOF profiles (solid light blue), reflecting how the FOF algorithm assigns particles to subhalos, leading to a loss of material from the central profile at large radii. Bottom: Comparison of central galaxy profiles measured with circular (light blue) and elliptical (dark blue) annuli using FOF particles, and with our mock observing routine including satellite masking (red). Elliptical annuli yield systematically higher central densities (by nearly $50\%$ at the outskirts) compared to circular annuli. Satellite masking (red) raises the density at small radii and lowers it at large radii relative to the FOF-based subtraction (dark blue), highlighting differences in how subhalos are treated. Overall, these comparisons show that choices of particle selection, annulus geometry, and satellite treatment each produce systematic differences in $\Sigma_\ast$ profiles.
• Figure 4: Top: Comparison of median stellar surface density profiles form HSC (black) and mock-observed central galaxies from TNG50, TNG100, and rescaled TNG300, shown in three stellar mass bins. Stellar mass is defined as the mass within a $100$ kpc semi-major axis of the galaxy center. Lighter shaded regions show the 16th and 84th percentile scatter. Bottom The simulated profiles compared to the observed profiles. The vertical line at 100 kpc shows the maximum extent where we are confident in the background subtraction of our HSC profiles huang.2018_photo_perform. TNG50 matches the observations best in all mass bins.
• Figure 5: Slopes and scatters of the SHMR in TNG100 for various stellar mass definitions, showing which definitions are the best tracers of halo mass. The bottom-left triangle shows the SHMR slopes, with aperture mass definitions (which include the inner most core) being found in the left most column. The top-right triangle shows the scatter in $M_{\rm{vir}}$, with aperture mass definitions along the top row. We find the stellar masses with the smallest scatter in halo mass correspond to inner radii of $\sim70 - 200$ kpc and outer radii of $\sim125 - 500$ kpc. This indicates that stellar mass measured in annuli towards the outskirts, rather than total aperture mass, provides the most robust tracer of halo mass.