There’s life in the old galaxies yet

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. NASA, Carnegie Observatories, Chandra

But astronomers studying the 100 brightest galaxies in the cluster Abell 2104 have found six active supermassive black holes rather than the one they had expected. Paul Martini, John S. Mulchaey, and their collaborators, all of Carnegie Observatories in Pasadena, Calif., examined the cluster with the orbiting Chandra X-ray Observatory. They found several areas with concentrations of high-energy X rays, which are signposts of material falling into a central, supermassive black hole. Visible-light spectra confirmed that the X rays emanate from galaxies within the cluster, Martini and his colleagues report in the Sept. 10 Astrophysical Journal Letters.

If other clusters also have more supermassive black holes than expected, it could mean that old galaxies within clusters either retain gas at their cores or funnel gas toward their centers, Mulchaey notes. Funneling might occur when a galaxy is stirred up by either a close encounter with a neighbor or its own movement toward the cluster’s center.

A greater than expected number of black holes might also mean that supermassive black holes in clusters fizzle out in less than 100 billion years, the standard estimate.

Black holes that don’t last that long or that wax and wane wouldn’t need as much gas to power them.


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