Entropy plateaus can emerge from gas replacement at a characteristic halo mass in simulated groups and clusters of galaxies

TL;DR

This study uses cosmological simulations to show that entropy plateaus in galaxy groups and clusters emerge at a characteristic halo mass due to AGN feedback, which redistributes low-entropy gas and affects the IGM's thermal structure.

Contribution

It demonstrates that entropy plateaus form at a specific halo mass and links their development to AGN feedback, providing new insights into IGM thermodynamics in galaxy systems.

Findings

Entropy profiles flatten at the virial radius around 10^{12} M_sun halos.

Entropy plateaus extend inward as halos grow into groups and clusters.

AGN feedback is the primary driver of entropy buildup by redistributing low-entropy gas.

Abstract

The evolution of the intergalactic medium (IGM) is influenced by gravitational collapse, radiative cooling, and baryonic feedback. Using cosmological hydrodynamic zoom-in simulations of a $8.83 \times 10^{12}$ M$_\odot$ group and a $2.92 \times 10^{14}$ M$_\odot$ cluster at $z=0$, we investigate the emergence of entropy plateaus and their connection to feedback mechanisms. This set-up uses the SWIFT-EAGLE model with three resolutions, down to an initial particle gas mass of $2.29 \times 10^5$ M$_\odot$ and $1.23 \times 10^6$ M$_\odot$ for dark matter. We find that, when halos reach the characteristic mass of $\sim 10^{12}$ M$_{\odot}$, their entropy profiles flatten at the virial radius, marking a transition from supernova to AGN feedback-driven regulation. As halos grow into groups ($\sim 10^{13}$ M$_{\odot}$), the entropy plateau extends inward and isentropic cores form in massive systems ($\sim 10^{14}$ M$_{\odot}$). By tracking the Lagrangian history of gas particles, we demonstrate that this entropy buildup is primarily driven by AGN feedback, which efficiently removes low-entropy gas from progenitors of groups and clusters, redistributing it throughout the IGM before falling into the core. Recent observations of X-GAP groups reveal large entropy excesses and plateaus, in line with our findings and in contrast to the power-law-like profiles of most previous observations. While entropy plateaus and large entropy excesses may be observationally confirmed in unbiased samples, reproducing the full diversity of entropy profiles remains an outstanding challenge for next-generation feedback models. Our results suggest that current feedback models may be overly efficient in expelling low-entropy gas from the potential cool-core progenitors, disrupting the balance between heating and cooling required for long-lived cool cores.