Astronomers identify most distant galaxy cluster

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Subaru, NASA, JPL-Caltech.

UCR Strategic Communications.

RIVERSIDE, Calif. — Bahram Mobasher, a professor of physics and astronomy at the University of California, Riverside, is a member of an international team of astronomers that has uncovered a burgeoning galactic metropolis, the most distant known in the early universe. This ancient collection of galaxies presumably grew into a modern galaxy cluster similar to the massive ones seen today. The developing cluster, named COSMOS-AzTEC3, was discovered and characterized by multi-wavelength telescopes, including NASA's Spitzer, Chandra and Hubble space telescopes, and the ground-based W.M. Keck Observatory and Japan's Subaru Telescope.

"This is a galaxy cluster in the process of formation — a proto-cluster — more than 12 billion years ago," Mobasher said. "This proto-cluster was formed about one billion years after the Big Bang. Study of such structures reveals how galaxies came together and merged to form larger galaxies."

In the research project, Mobasher helped to identify member galaxies of this cluster and measure their distances and velocities. He also calculated when the proto-cluster was formed. To measure the spectra of candidates in the cluster, he secured observing time on the Keck Telescope in Hawaii.

Study results appear in the Jan. 13 issue of Nature.

Besides being the most distant massive proto-cluster known, COSMOS-AzTEC3 is one of the youngest because it is being seen when the universe itself was young. The cluster is roughly 12.6 billion light-years away from Earth. Our universe is estimated to be 13.7 billion years old. Previously, more mature versions of these clusters had been spotted at 10 billion light-years away.

The team of astronomers also found that this cluster is buzzing with extreme bursts of star formation and one enormous feeding black hole.

"We think the starbursts and black holes are the seeds of the cluster," said Peter Capak of NASA's Spitzer Science Center at the California Institute of Technology in Pasadena, and the first author of the research paper. "These seeds will eventually grow into a giant, central galaxy that will dominate the cluster — a trait found in modern-day galaxy clusters."

Most galaxies in our universe are bound together into clusters that dot the cosmic landscape like urban sprawls, usually centered around one old, monstrous galaxy containing a massive black hole. Astronomers thought that primitive versions of these clusters, still forming and clumping together, should exist in the early universe. But locating one proved difficult — until now.

The researchers first used the Chandra X-ray Observatory and the United Kingdom's James Clerk Maxwell Telescope on Mauna Kea, Hawaii, to search for the black holes and bursts of star formation needed to form the massive galaxies at the centers of modern galaxy cities. The astronomers then used Hubble and the Subaru telescopes to estimate the distances to these objects, and look for higher densities of galaxies around them. Finally, the Keck telescope was used to confirm that these galaxies were at the same distance and part of the same galactic sprawl.

Once the scientists found this lumping of galaxies, they measured the combined mass with the help of Spitzer. At this distance the optical light from stars is shifted, or stretched, to infrared wavelengths that can only be observed in outer space by Spitzer. The lump sum of the mass turned out to be a minimum of 400 billion suns – enough to indicate that the astronomers had indeed uncovered a massive proto-cluster. The Spitzer observations also helped confirm a massive galaxy at the center of the cluster was forming stars at an impressive rate.

Chandra X-ray observations were used to find and characterize the whopping black hole with a mass of more than 30 million suns. Massive black holes are common in present-day galaxy clusters, but this is the first time a feeding black hole of this heft has been linked to a cluster that is so young.

Finally, the Institut de Radioastronomie Millimétrique's interferometer telescope in France and 30-meter telescope in Spain, along with the National Radio Astronomy Observatory's Very Large Array telescope in New Mexico, measured the amount of gas, or fuel for future star formation, in the cluster. The results indicate the cluster will keep growing into a modern city of galaxies.

COSMOS-AzTEC3, located in the constellation Sextans, is named after the region where it was found, called COSMOS after the Cosmic Evolution Survey. AzTEC is the name of the camera used on the James Clerk Maxwell Telescope – this camera is now on its way to the Large Millimeter Telescope located in Mexico's Puebla state.

Source: University of California - Riverside