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

TL;DR

This work assesses how recent mass accretion shapes the thermodynamical structure of the ICM by analyzing a cooling-only hydrodynamical cosmological simulation. Leveraging azimuthal median radial profiles and a thorough stacking and fitting framework, the study demonstrates that recent accretion mainly lowers central gas densities (DM remains largely unchanged), shifts the locations of key entropy, temperature, and pressure features inward, and imprints discernible trends in the fit parameters of standard density and entropy models. The findings highlight the potential of thermodynamic profiles as tracers of cluster growth history, while noting that feedback processes and projection effects must be incorporated for direct observational comparisons. Overall, the paper provides a clear, quantitative link between mass assembly, ICM structure, and the interpretation of observed cluster thermodynamics.

Abstract

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.

Figures (13)

• Figure 1: Comparison between mean (red solid line), median (cyan solid line) and mode (orange dashed line) gas density profiles in two galaxy clusters (corresponding to the two panels). In addition to the standard mean profiles, also profiles with substructures having been masked are shown as the dashed, red lines. Light gray lines indicate the individual directional profiles, while cyan dotted lines are the $(16-84)\%$ percentiles at each $r$ to give a better idea of the width of the density distribution. Dark blue dotted lines indicate the thresholds for the substructure cleaning algorithm from which the red dashed line has been obtained. The insets provide a zoomed-in view of two regions to highlight the differences among the profiles. The blue filled line at the right of the right-hand panel inset depicts the distribution of values of density at $r \sim 3 R_\mathrm{vir}$, to highlight its bimodality.
• Figure 2: Mass distribution for the highest accreting third (green) and lowest accreting third (red) of the sample. Left-hand side panel: gas density. Central panel: DM density. Right-hand side panel: baryon depletion. Within each column, the top panel presents the profiles stacked over each subsample, together with the whole population (gray lines). The second panel shows each class normalised by the ensemble average. The third panel contains the logarithmic slopes of the profiles, together with an indication of the radius of minimum slope. Last, the bottom panels show the correlation between the profiles and accretion rate $\Gamma_{200m}$ controlling for $M_{200m}$.
• Figure 3: Thermodynamical state of the highest-$\Gamma_{200m}$ third (green) and the lowest-$\Gamma_{200m}$ third (red) of the sample. Left-hand side panel: temperature. Central panel: entropy. Right-hand side panel: thermal pressure. Vertical panels within each column contain the same information as in Fig. \ref{['fig:density_profiles']}.
• Figure 4: Effect of individual indicators of assembly state on the entropy profiles. Top panel: Spearman (partial) correlation coefficients of each indicator and the value of the profiles at each $r/R_{200m}$. Higher magnitudes (either positive or negative) indicate larger influence of the value of the indicator on the profile at this particular radius. Middle panel: effect of selecting clusters based on each parameter on the location and the depth of the steepest logarithmic slope of the entropy profile. Crosses correspond to the profile stacked over the one-third most relaxed subsample (according to the given indicator), while filled dots correspond to the most disturbed third. Bottom panel: Similar to the middle panel, but with the location and height of the entropy peak. The blue regions in the middle and bottom panel indicate the $68\%$ confidence region for the determination of the corresponding locations over each of the $\Delta_r$-based subsamples (as an example), obtained by bootstrap resampling.
• Figure 5: Effect of individual indicators of assembly state on the pressure profiles. Top panel: similar to the top panel of Fig. \ref{['fig:entropy_indicators']}, with the pressure profiles. Bottom panel: similar to the middle panel of Fig. \ref{['fig:entropy_indicators']}, with the pressure profiles. The colour code and other figure elements is kept the same as in the aforementioned figure.