21 January: Scott Kay

Speaker: Scott Kay (University of Manchester)

Date: Wednesday 21 January 2026

Time: 15:00

Location: Physics 3.21 Berry

The Entropy Core Problem in Simulations of Galaxy Groups and Clusters

It has long been established that the hot, X-ray emitting intrahalo gas in groups and clusters must have more entropy than what is generated from gravitational collapse and must therefore be affected by non-gravitational physics. Early observational evidence for this excess entropy came from the X-ray luminosity-temperature scaling relation, subsequently confirmed from measurements of the entropy profiles on different mass scales. While early theoretical ideas suggested that the gas may have been pre-heated (by feedback processes) before the object’s collapse, this scenario was subsequently ruled out as it predicted entropy cores, at odds with many of the observed (power-law-like) entropy profiles. A more physical explanation for the excess entropy, validated by simulations, is that the low entropy gas has been removed from the hot phase, either by cooling or from being ejected from progenitor haloes by the feedback at high redshift. These initial simulations with feedback were also able to broadly reproduce the observed X-ray entropy profiles. However, in recent years, the entropy core problem has returned in modern, high-resolution simulations that successfully reproduce the observed galaxy population (including the quenching of star formation in massive galaxies by AGN). In this talk, I will show our recent simulation results that investigate this problem further, how we think it occurs, then briefly discuss some unresolved issues that may contribute to finally solving this puzzle.