Composite of x-ray and visible light views of a distant cluster of galaxies

This image superimposes an X-ray image of a distant cluster (the blue pixellated image, from ESA's XMM-Newton satellite) on top of a ground-based view of the sky (from the Canada France Hawaii Telescope). Some of the brightest X-ray objects are galaxies with brilliant centres powered by supermassive black holes. The cluster at the centre of the picture shows up as an extended patch of X-ray emission, coming from hot gas.