Summary

After having studied this lecture, you should be able to

• Define what is a cluster and a group of galaxies by listing some of their properties.
• Explain how galaxy motions suggest the presence of dark matter in clusters.
• Explain how X-ray observations suggest the presence of dark matter in clusters.
• Explain the origin of the high temperature of the cluster gas.
• Explain how we know that most of the baryons in a cluster are in hot gas, and not in stars.
• Explain why the high observed metallicity of the cluster gas suggests that some kind of stellar feedback mechanism prevented most of the gas turn into stars before it fell into the cluster.
• Explain how clusters have been used to estimate the mean dark matter density of the Universe.