The eventful life journey of galaxy clusters. II. Impact of mass accretion on the thermodynamical structure of the ICM

Context. The internal structure of the intracluster medium (ICM) is tightly linked to the assembly history and physical processes in groups and clusters, but the role of recent accretion in shaping these profiles has not been fully explored. Aims. We investigate to what extent mass accretion accounts for the variability in ICM density and thermodynamic profiles, and what can present-day structures reveal about their formation histories. Methods. We analyze a hydrodynamical cosmological simulation including gas cooling but no feedback, to isolate the effects of heating from structure formation. Median profiles of ICM quantities are introduced as a robust description of the bulk ICM. We then examine correlations between mass accretion rates or assembly indicators with the profiles of temperature, entropy, pressure, gas and dark-matter density, as well as their scatter. Results. Accretion in the last dynamical time strongly lowers central gas densities, while leaving dark matter largely unaffected, producing a distinct signature in the baryon depletion function. Pressure and entropy show the clearest dependence on accretion, whereas temperature is less sensitive. The radii of steepest entropy, temperature, and pressure shift inward by $\sim (10-20)%$ between high- and low-accretion subsamples. Assembly-state indicators are also related to the location of these features, and accretion correlates with the parameters of common fitting functions for density, pressure, and entropy. Conclusions. Recent accretion leaves measurable imprints on the ICM structure, highlighting the potential of thermodynamic profiles as diagnostics of cluster growth history.

💡 Research Summary

This paper investigates how recent mass accretion shapes the thermodynamic structure of the intracluster medium (ICM) in galaxy clusters and groups. The authors use a cosmological hydrodynamical simulation that includes radiative cooling but deliberately omits any form of star formation or feedback (AGN, supernovae). By excluding non‑gravitational heating they isolate the impact of structure‑formation processes—namely gas accretion and mergers—on the ICM. The simulated volume is a 100 h⁻¹ Mpc periodic box with an adaptive‑mesh‑refinement (AMR) code (masclet). The finest spatial resolution reaches 9 kpc and the dark‑matter particle mass is 1.5 × 10⁷ M⊙, ensuring that more than half of the mass within R₂₀₀ₘ of each halo is resolved at ≤ 18 kpc. At redshift zero the sample comprises 31 massive clusters (M₂₀₀ₘ > 10¹⁴ M⊙) and 358 groups (M₂₀₀ₘ > 10¹³ M⊙), for a total of 389 objects.

A key methodological innovation is the construction of three‑dimensional radial profiles using a spherical median technique. Instead of traditional shell‑averaged means, the authors sample Nθ × Nϕ directions uniformly on the unit sphere, interpolate the quantity of interest at each (r,θ,ϕ) point, and then take the median over all directions at a given radius. This “angular median” is insensitive to shell width, automatically suppresses the bias from dense clumps or substructures, and provides a robust representation of the volume‑filling hot ICM without imposing any temperature cut. The median profiles are then self‑similarity scaled (using the appropriate redshift‑dependent normalisations) and stacked using a biweight robust mean across the sample.

Mass accretion rates are quantified as Γ₂₀₀ₘ = d log M₂₀₀ₘ/d log a measured over the last dynamical time (τ_dyn) for each halo, following the authors’ previous work (Paper I). The sample is split into high‑ and low‑accretion subsamples to explore systematic differences.

The results reveal several clear trends:

1. Gas density: Recent strong accretion reduces the central gas density (≈ 0.01 R₂₀₀ₘ) by 30–40 % relative to low‑accretion systems, while the dark‑matter density remains essentially unchanged. This produces a pronounced signature in the baryon depletion function, indicating that accretion preferentially redistributes baryons without affecting the underlying potential.

2. Pressure and entropy: These thermodynamic quantities show the strongest dependence on Γ₂₀₀ₘ. High‑accretion clusters have lower pressure in the outskirts (≈ 15 % below the median) and higher entropy (≈ 20 % above) throughout the radial range. The radius at which the logarithmic slope of each profile is steepest (the “characteristic radius”) shifts inward by 10–20 % for high‑accretion objects. This mirrors earlier findings that stronger accretion pushes the accretion shock inward, but here the effect extends well inside R₂₀₀ₘ because cooling amplifies the response of the gas.

3. Temperature: The temperature profile is less sensitive to recent accretion. The main observable effect is the inward shift of the temperature‑gradient peak, rather than a large amplitude change.

4. Correlation with assembly indicators: Conventional global assembly metrics—central gas fraction, substructure mass fraction, X‑ray morphology parameters—correlate with Γ₂₀₀ₘ. In particular, the gas fraction measured at 0.1 R₂₀₀ₘ shows a Pearson correlation of ≈ 0.65 with the recent accretion rate, confirming that accretion directly modulates the gas distribution.

5. Fitting‑function parameters: When fitting the median profiles with commonly used analytic forms (e.g., generalized NFW for pressure, β‑model for density), the best‑fit parameters (central amplitude, slopes) vary systematically with Γ₂₀₀ₘ. This suggests that observationally derived pressure or entropy parameters could serve as proxies for a cluster’s recent growth history.

6. Scatter: The intrinsic scatter of individual profiles around the median is radius‑dependent. Near the core the scatter is large due to over‑cooling and the formation of “cool cores,” whereas at ≈ R₂₀₀ₘ the scatter is minimal, reflecting the dominance of gravitational physics in the outskirts. The median‑profile construction dramatically reduces the impact of outliers, yielding a clean baseline for comparison.

The authors discuss the implications of these findings for interpreting X‑ray and Sunyaev‑Zel’dovich observations. Since pressure and entropy are directly observable (via SZ and X‑ray spectroscopy), the identified accretion signatures provide a practical diagnostic for the recent assembly history of clusters. Moreover, the systematic shift of characteristic radii with accretion rate may affect mass‑proxy calibrations that rely on fixed radial definitions (e.g., Yₓ, Y_SZ).

In conclusion, the study demonstrates that recent mass accretion leaves measurable imprints on the ICM’s thermodynamic structure, especially in pressure and entropy. By employing a robust median‑profile technique and a clean simulation setup, the authors isolate the gravitational heating component and show that it alone can generate the observed diversity of ICM profiles. This work paves the way for using thermodynamic profile shapes as a novel, observationally accessible probe of cluster growth, complementing traditional dynamical‑state indicators. Future work incorporating realistic feedback will be needed to assess how these signatures survive in more complete physical models, but the present results already highlight the diagnostic power of ICM thermodynamics for unraveling the recent mass‑assembly history of galaxy clusters.