Astronomers find galaxy cluster obscured by quasar

Search is on for more galaxy clusters hidden by very bright, active supermassive black holes.

Astronomers have discovered a large galaxy cluster with a mass of approximately 690 trillion suns that until now was obscured by an extremely bright quasar, an active supermassive black hole feeding on material surrounding it.

Composed of several hundred individual galaxies, the cluster is located about 2.4 billion light years from Earth and surrounds the quasar.

Designated PKS1353-341, the quasar, which is 46 billion times brighter than our Sun, was long thought to be alone in its region of space. It is surrounded by a huge disk of swirling material, of which large chunks are falling into it and in the process radiating high levels of energy as light.

“This might be a short-lived phase that clusters go through, where the central black hole has a quick meal, gets bright, and then fades away again. This could be a blip that we just happened to see. In a million years, this might look like a diffuse fuzzball,” explained Michael McDonald of MIT‘s Kavli Institute for Astrophysics and Space Research.

The discovery of the galaxy cluster suggests other, similar clusters could be hiding behind extremely bright objects. Such clusters provide important information about the amount of matter in the universe and the rate at which the universe is expanding, which is why astronomers are now searching for them.

One reason scientists missed this large cluster is their assumption that clusters appear “fluffy” and give off diffuse X-ray signals, very unlike quasars, which are bright, single-point sources.

“This idea that you could have a rapidly accreting black hole at the center of a cluster–we didn’t think that was something that happened in nature,” McDonald said.

To find more hidden clusters, he and fellow researchers set up a survey titled Clusters Hiding in Plain Sight (CHiPS), which involved looking at archival X-ray images of very bright objects. They then followed up by studying these objects using the Magellan Telescope, an optical observatory in Chile.

If the Magellan observations revealed more galaxies than expected surrounding the bright object, they then observed the point source using the space-based Chandra X-ray Observatory.

“Some 90 percent of these sources turned out not to be clusters,” McDonald said.

The CHiPS survey did find one new galaxy cluster obscured by a very bright supermassive black hole.

Findings of the study have been published in the Astrophysical Journal.


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