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Entropy plateaus can emerge from gas replacement at a characteristic halo mass in simulated groups and clusters of galaxies
Edoardo Altamura, Scott T. Kay, Joop Schaye, Ian G. McCarthy, Matthieu Schaller
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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 ×10¹² M_⊙ group and a 2.92 ×10¹⁴ M_⊙ 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 ×10⁵ M_⊙ and 1.23 ×10⁶ M_⊙ for dark matter. We find that, when halos reach the characteristic mass of ∼10¹² M_⊙, their entropy profiles flatten at the virial radius, marking a transition from supernova to AGN feedback-driven regulation. As halos grow into groups (∼10¹³ M_⊙), the entropy plateau extends inward and isentropic cores form in massive systems (∼10¹⁴ M_⊙). 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.
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