Astronomers eclipse record for most distant massive object

An international team of astronomers, including a U-M researcher, used the world's largest X-ray and optical telescopes to spot the most distant massive object ever detected—a cluster of galaxies nine billion light years distant from Earth.

The cluster is so far away that the light detected by the team is much older than the Earth itself. The galaxy cluster, if it still is there, would be at least 11 billion years old now.

Courtesy Department of Astronomy

"By capturing this ancient, 9-billion-year-old light, we have a snapshot of the universe at a youthful age of less than 5 billion years, which is about one-third of the present age," says project leader Christopher Mullis, a research fellow in the Department of Astronomy.

As exciting as it is to break a record, it's also an important cosmological finding. "Just a few years ago, astronomers did not believe structures like this even existed at such an early time," Mullis says. This galaxy cluster, which is being seen as it appeared about
2 billion years after its formation, is well organized and mature, he says. Although it is very far back in time, it looks as if this structure had formed in a way that is consistent with more recent structures.

"Even at this early stage in cosmic history, this appears already as a mature, fully assembled structure which implies that this is an old cluster in a young universe," says European Southern Observatory astronomer Piero Rosati, who collaborated on the study.

The record-breaking galaxy cluster also was a somewhat surprising find for the team, which was testing a new approach to hunting distant objects. "Basically we stepped up to the plate for our first time at bat with this new system, and we hit a home run," Mullis says.

Mullis and his colleagues started their search by combing through archives of old images from the European Space Agency's orbiting X-ray observatory, XMM-Newton, looking for diffuse X-ray sources that previously had not been studied. Cluster galaxies shine brightly in optical light, but they also emit strong X-ray signals resulting from very hot gas that envelopes the cluster.

The record-breaking cluster initially turned up small but distinct, off-center in an image made by another team.

The X-ray image of the distant cluster is comprised of just 280 photons—individual parcels of light—collected during a 12.5-hour exposure. By comparison, on a sunny day the human eye is flooded by about 10 quadrillion photons per second.

With this distant cluster candidate and dozens of others culled from the X-ray archive, Mullis and his team turned to one of the world's largest optical telescopes, the European Southern Observatory's Very Large Telescope, located in the Atacama Desert, Chile. They took a series of relatively quick exposures of the candidates with red and blue filters on the telescope.

What Mullis and his Italian and German collaborators were looking for at each of the candidate spots were very red galaxies, indicating light that has traveled for an extremely long time to reach Earth. "The redder the better," Mullis says. Almost immediately, they turned up this cluster of red objects that seemed to be beyond the previous distance record.

Mullis and his team will broaden the search to find more super-distant galaxy clusters with this new approach. They also plan to go back and take longer optical and X-ray telescope exposures of the cluster to get a better sense of its features.

Mullis will present this finding at an international astronomy conference in Hawaii focused on connecting galaxy clusters to the underlying physics of space time and gravity. The meeting is being organized by U-M physics professor Gus Evrard, and sponsored in part by the Michigan Center for Theoretical Physics.

A paper by Mullis and his team also will appear in an upcoming issue of The Astrophysical Journal.

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