According to conventional wisdom, clusters of elderly galaxies settled into retirement long ago. With most of their cold gas already having coalesced into stars, these clusters can no longer make new stars or keep supermassive black holes active.
TRACERS. X-ray image of the Abell 2104 cluster of galaxies overlaid with a visible-light image. Peripheral blue patches are X rays emitted by material falling into supermassive black holes.
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