Keep Calm and Baryon: The Distribution of Baryons and Dark Matter on Cosmic Scales

Abstract

How are baryons distributed in the universe? Where, and in what abundances, are they located in large, virialized systems such as galaxies, groups, and clusters? Here we determine the distribution of baryons, relative to dark matter, in systems ranging in size from galaxies to groups, clusters, and large-scale structure. We use observed X-ray and SZ measurements of the hot intracluster medium (ICM) to determine the hot gas contribution in groups and clusters; weak lensing and optical constraints on the cluster stellar fraction; and absorption measurements of the cool circumgalactic medium (CGM) to determine the gas mass in galaxies. Using direct observations when possible, and extrapolations of observed density pro les when necessary, we show that the baryon content within the virial radius is consistent with the cosmic baryon fraction (0:164 0:004) for systems ranging over three orders of magnitude in mass, from galaxies to groups and massive clusters. The baryon distribution is less extended in massive systems and more broadly distributed in low-mass systems. Averaged on scales larger than the virial radius, the baryonic mass is a strong tracer of the underlying dark matter distribution, and the dark matter, stellar, and gaseous components of groups, clusters, and large-scale structure may be comprised only of the contributions from the individual constituent galaxies of these larger systems. Baryons, which initially fell into the gravitational potentials formed by dark matter, have not been removed significantly from these systems, but remain in roughly the cosmic fraction in galaxies, groups, clusters, and large-scale structure.