Clusters have been growing over the last 10 billion years through the continuing infall of material, mostly clumps of dark matter, primarily along the filamentary structures (Figure 1). In some clumps, the accompanying baryonic matter has cooled sufficiently for stars and galaxies to form. Once these galaxies reach the denser environment of the cluster cores, they are scattered by local irregularities in the gravitational potential. Their velocities begin to isotropize and develop a randomized velocity structure typical of systems in virial equilibrium. The “virial radius” for rich clusters, within which there has been sufficient time to approach this state, is typically about 2 Mpc for mass > 1015 M⊙ clusters, and virial velocities are of order 1000 km/s.
When the infalling baryons are still in a diffuse state, they experience a quite different fate. An “accretion shock,” seen in simulations but not yet in observations, forms about 2 times further out than the virial radius from the cluster center. As the infalling diffuse material encounters the shock, their infall energy is almost entirely converted into heat. This brings the gas to an equivalent “virial temperature” of approximately 108 K, thus forming the ICM plasma. As the stars within cluster galaxies continue to evolve, winds and shocks transport material into the ICM, enriching it with what astronomers call “metals,” the elements from C to Fe that were produced in stellar nucleosynthesis.
Given sufficient time without major perturbations, clusters can take on a “relaxed” state, with the infall of gas and galaxies contributing to the growth of a massive central galaxy, 1013 M⊙. The radiative cooling time in the inner regions is shorter than the age of the cluster, and the temperatures can drop towards the cluster center. Such relaxed clusters have high X-ray luminosities and are prominent in cluster surveys. They are recognized by their bright, central cores, their regular velocity distributions, and their generally symmetric appearance. This apparent relaxation is deceiving.
Occasionally, the infalling clumps of gas and baryons are a substantial fraction of the cluster mass. In that case, there is little prompt heating of the infalling baryons and a “cold” (i.e., only ∼ 107 K!) clump of gas can penetrate deep into the cluster. Many such structures are identified by their bright appearance and sharp edges. They are distinguished from shocks because they are in pressure equilibrium with the surrounding hot ICM. While remaining cold, the clump may also generate a bow shock and heat the surrounding medium. If the intruder is massive enough, it can even set off “sloshing” motions in the ICM baryons, as they are displaced from, and then restored by, the underlying dark matter core. In these cases, spiral waves can be set up, as can be dramatically seen in Figure 2. Here, a highly enhanced X-ray image of the nearby Perseus cluster of galaxies reveals the effects of a likely off-center encounter with a 1014 M⊙ clump approximately 2.5 billion years ago. In this case, some of the energy has also been transferred into the cosmic rays, leading to a so-called synchrotron “mini-halo” shown in contours. The sloshing motions can help mix the cooler and hotter gas, redistribute metals throughout the cluster, and induce turbulence that eventually dissipates the energy into heat. Buried deep in the center is also an AGN associated with the bright central galaxy NGC 1275. Its ejections evacuate bubbles in the X-ray gas, creating dark patches.
Figure 2. Inner 400 kpc of Perseus cluster in X-rays, filtered to show fine structure, courtesy of Stephen Walker, NASA Goddard, superposed with r-band image from the Sloan Digital Sky Survey and radio contours of the “mini-halo,” courtesy of Marie-Lou Gendron-Marsolais, University of Montreal.