The Baryon Content of Galaxy Systems: Observations and Simulations

Abstract

The distribution of baryons in the universe tells us about the evolution and processes occurring within galaxies and clusters. Attempts to probe the gas within these structures have revealed a significantly lower baryon content than the cosmic fraction of \(\sim 16.4\%\) (as determined by CMB observations). However, most of these observational methods (namely X-ray and SZ) are only sensitive enough to measure a small fraction of the whole virialized volume. By using a combination of observational data and extrapolation to larger radii, in addition to a few particularly deep observations which extend to the virial radius, we may successfully account for these ``missing baryons." We also analyze galaxy clusters within several high-resolution cosmological simulations to see how well they reproduce observed trends in gas, stellar and dark matter distribution. We find that for systems with masses ranging from \(10^{13} - 10^{15} M_{\odot}\), the baryon content achieves the cosmic fraction at slightly past the virial radius. Expelled from the cores of their clusters by feedback processes (shock heating, supernovae and potentially AGN), these baryons reside in the halos in the form of hot plasma. Simulations confirm the existence of significant amounts of gas on these scales. However, overall consistency with observations varies depending on the specific physics included in the simulations, and how it is implemented.