EAGLE-like simulation models do not solve the entropy core problem in groups and clusters of galaxies

TL;DR

High-resolution cosmological simulations with the EAGLE model consistently predict overly high entropy in galaxy groups and clusters' cores, conflicting with observations, and varying physical or numerical parameters does not fully resolve this discrepancy.

Contribution

This study systematically tests the impact of different sub-grid physics and feedback schemes on the entropy profiles in simulated galaxy groups and clusters.

Findings

Simulations predict too high core entropy compared to observations.

Removing artificial conduction, metal cooling, or AGN feedback lowers entropy but does not match observed profiles.

Bipolar AGN heating yields higher and more uniform entropy distributions.

Abstract

Recent high-resolution cosmological hydrodynamic simulations run with a variety of codes systematically predict large amounts of entropy in the intra-cluster medium at low redshift, leading to flat entropy profiles and a suppressed cool-core population. This prediction is at odds with X-ray observations of groups and clusters. We use a new implementation of the EAGLE galaxy formation model to investigate the sensitivity of the central entropy and the shape of the profiles to changes in the sub-grid model applied to a suite of zoom-in cosmological simulations of a group of mass $M_{500} = 8.8 \times 10^{12}~{\rm M}_\odot$ and a cluster of mass $2.9 \times 10^{14}~{\rm M}_\odot$. Using our reference model, calibrated to match the stellar mass function of field galaxies, we confirm that our simulated groups and clusters contain hot gas with too high entropy in their cores. Additional simulations run without artificial conduction, metal cooling or AGN feedback produce lower entropy levels but still fail to reproduce observed profiles. Conversely, the two objects run without supernova feedback show a significant entropy increase which can be attributed to excessive cooling and star formation. Varying the AGN heating temperature does not greatly affect the profile shape, but only the overall normalisation. Finally, we compared runs with four AGN heating schemes and obtained similar profiles, with the exception of bipolar AGN heating, which produces a higher and more uniform entropy distribution. Our study leaves open the question of whether the entropy core problem in simulations, and particularly the lack of power-law cool-core profiles, arise from incorrect physical assumptions, missing physical processes, or insufficient numerical resolution.