These two nearby galaxy clusters, known as CL 0542-4100 and CL 0848.6+4453, are part of a sample used to count the fraction of galaxies with rapidly growing black holes, also known as active galactic nuclei (AGN). Red corresponds to low-energy X-rays, the green to intermediate-energy, and the blue to high-energy X-rays. A new study shows that nearby, older clusters like these contain far less AGN than young, more distant clusters. Credit: NASA/CXC/OSU/P. Martini et al.
