The Three Hundred Project: deducing the stellar splashback structure of galaxy clusters from their orbiting profiles

TL;DR

The paper investigates a physically meaningful boundary of galaxy clusters—the splashback radius—by comparing the stellar and dark matter outer profiles in hydrodynamical simulations from The Three Hundred Project. It decomposes material into orbiting and infalling components and fits their density profiles with Diemer-style functions, revealing that $r_{ m t}$ is coincident for stars and DM while the stellar edge is steeper. Both components share the same $r_{ m t}-\Gamma$ relation, enabling inference of recent mass growth from observations. Projection-based fits to total stellar profiles can recover $R_{ m t}$ with modest scatter ($\sim 0.3\,R_{200\mathrm{m}}$), validating stellar tracers like the ICL as observable proxies of cluster boundaries and assembly history.

Abstract

We examine the splashback structure of galaxy clusters using hydrodynamical simulations from the GIZMO run of The Three Hundred Project, focusing on the relationship between the stellar and dark matter components. We dynamically decompose clusters into orbiting and infalling material and fit their density profiles. We find that the truncation radius $r_{\mathrm{t}}$, associated with the splashback feature, coincides for stars and dark matter, but the stellar profile exhibits a systematically steeper decline. Both components follow a consistent $r_{\mathrm{t}}{-}Γ$ relation, where $Γ$ is the mass accretion rate, which suggests that stellar profiles can be used to infer recent cluster mass growth. We also find that the normalisation of the density profile of infalling material correlates with $Γ$, and that stellar and dark matter scale radii coincide when measured non-parametrically. By fitting stellar profiles in projection, we show that $r_{\mathrm{t}}$ can, in principle, be recovered observationally, with a typical scatter of $\sim 0.3\,R_{200\mathrm{m}}$. Our results demonstrate that the splashback feature in the stellar component provides a viable proxy for the cluster's physical boundary and recent growth by mass accretion, offering a complementary observable tracer to satellite galaxies and weak lensing.

Figures (15)

• Figure 1: Cluster 001 from the GIZMO_3k run, decomposed into orbiting (yellow-purple) and infalling (black) particles. The top row shows the $x$--$y$ projection of the dark matter (left) and stars (right), while the bottom row shows their radial phase space. The cluster's $R_{200{\rm c}}$ and $R_{200{\rm m}}$ are marked.
• Figure 2: The density profiles of the orbiting and infalling components, fit using Equations \ref{['eq:orbiting-fitting-function-dm']}--\ref{['eq:infalling-fitting-function']}. The BCG component is confined to the inner regions, while the stellar "halo" takes on a similar shape to the orbiting profile of the dark matter. The logarithmic slope profiles are shown in the bottom panels.
• Figure 3: Comparison between $r_{\rm t}$ and $\beta$ obtained from fitting the dark matter and stellar orbiting profiles. The points are colored by accretion rate (Equation \ref{['eq:accretion-rate']}). The square points are the result of fitting the $\Gamma$-binned median profiles.
• Figure 4: Comparison between the radius of steepest slope, $r_{\rm steep}$ and $r_{\rm t}$ for the dark matter and stars. The points are colored by accretion rate (Equation \ref{['eq:accretion-rate']}). The square points are the result of fitting the $\Gamma$-binned median profiles. The black dashed lines are linear fits given by $r_{\rm steep}/R_{200{\rm m}}=0.83(r_{\rm t}/R_{200{\rm m}})+0.41$ for the dark matter and $r_{\rm steep}/R_{200{\rm m}}=0.83(r_{\rm t}/R_{200{\rm m}})+0.37$ for the stars.
• Figure 5: $r_{\rm steep}$ and $r_{\rm t}$ as a function of accretion rate (Equation \ref{['eq:accretion-rate']}). The light blue points are measurements for individual profiles, while the dark blue points are the median and standard deviation in equally-spaced bins in $\Gamma$. The red squares are from fitting the $\Gamma$-binned median profiles. The black dashed line in the top row is the relation from More2015, while in the bottom row it is the same relation rescaled using the best-fit $r_{\rm t}$--$r_{\rm steep}$ relation shown in Figure \ref{['fig:rsteep-vs-rt']}. The black dotted line is the relationship from Diemer2025.