Abstract

Only ~10 per cent of baryons in the Universe are in the form of stars, yet most models of luminous structure formation have concentrated on the properties of the stellar matter. In this talk I will focus on the “flip side” of galaxy formation and investigate the properties of the material that is not presently locked up in galaxies. This “by-product” of galaxy formation can be observed as an X-ray emitting plasma [the intracluster medium (ICM)] in groups and clusters. Since a significant fraction of this material has been processed through galaxies, observations of the ICM represent an orthogonal set of constraints on galaxy formation models.

I first review the state of the hot gas in massive galaxy clusters, where the observational X-ray data is of the highest quality. Using a simple model, I will argue that the energetics required to the establish the distribution of the gas in these systems is beyond the reach of present-day AGN and that a significant degree of so-called “preheating” is required. While preheating models successfully reproduce the present-day properties of the ICM , they have profound implications for the evolution of the ICM and the state of the gas in galaxies and galaxy groups – namely that there should be large numbers of very X-ray underluminous galaxies and groups. I will discuss the observational evidence for such systems. In the second part of the talk, I will present preliminary results from two new sets of cosmological hydrodynamic simulations (the OverWhelmingly Large Simulations, OWLS , and the Galaxies-Intergalactic Medium Interaction Calculation, GIMIC ). These simulations place the preheating scenario in a stronger physical context and make falsifiable predictions for upcoming surveys.