Reexamination of the Galaxy Formation–regulated Gas Evolution Model in Groups and Clusters

As an alternative explanation of the entropy excess and the steepening of theX-ray luminosity-temperature relation in groups and clusters, the galaxyformation-regulated gas evolution (GG) model proposed recently by Bryan makesan attempt to incorporate the formation of galaxies into the evolution of gaswithout additional heating by nongravitational processes. This seems to providea unified scheme for our understanding of the structures and evolution of bothgalaxies and gas in groups and clusters. In this paper, we present an extensivecomparison of the X-ray properties of groups and clusters predicted by the GGmodel and those revealed by current X-ray observations, using various largedata sources in the literature and also taking the observational selectioneffects into account. These include an independent check of the fundamentalworking hypothesis of the GG model, i.e., galaxy formation was less efficientin rich clusters than in groups, a new test of the radial gas distributionsrevealed by both the gas mass fraction and the X-ray surface brightnessprofiles, and an reexamination of the X-ray luminosity-temperature andentropy-temperature relations. In particular, it shows that the overall X-raysurface brightness profiles predicted by the GG model are very similar inshape, insensitive to the X-ray temperature, and the shallower X-ray surfacebrightness profiles seen at low-temperature systems may arise from the currentobservational selection effect. This can be used as the simplest approach todistinguishing between the GG model and the preheating scenario. The latteryields an intrinsically shallower gas distribution in groups than in richclusters.Comment: 30 pages, 10 figures, accepted for publication in Ap