Good CARMA contributes to detection of rare gravitational phenomenon

Astronomers spotted the cluster in a search using NASA’s Spitzer Space Telescope in combination with archival optical images taken as part of the National Optical Astronomy Observatory’s Deep Wide Field Survey at the Kitt Peak National Observatory in Tucson, Ariz. The combined images allowed them to see the cluster as a grouping of very red galaxies, indicating they are far away.

This unique system constitutes the most distant cluster known to “host” a giant gravitationally lensed arc. Finding this ancient gravitational arc may yield insight into how, during the first moments after the big bang, conditions were set up for the growth of hefty clusters in the early universe.

The arc was spotted in optical images of the cluster taken in 2010 by Hubble’s Advanced Camera for Surveys. The infrared capabilities of Hubble’s Wide Field Camera 3 (WFC3) helped provide a precise distance, confirming it to be one of the farthest clusters yet discovered.

Once the astronomers determined the cluster’s distance, they used Hubble, the CARMA radio telescope, and NASA’s Chandra X-ray Observatory to independently show that the galactic grouping is extremely massive.

CARMA is an excellent observatory for studying such a distant cluster of galaxies because it images the shadow of the cluster against the cosmic microwave background radiation, the fossil light from the big bang, said Tom Plagge, a UChicago postdoctoral research fellow and associate fellow of the Kavli Institute for Cosmological Physics. This signal is independent of the distance to the cluster, allowing CARMA to quickly and robustly detect massive clusters across the observable universe, if one knows where in the sky to look.

“It was hard to believe they had found such a high-mass cluster at so great a distance, but we knew that CARMA observations would quickly provide a good estimate of its mass,” Plagge said.

CARMA helped the astronomers determine the cluster’s mass by measuring how primordial light from the big bang was affected as it passed through the extremely hot, tenuous gas that permeates the grouping. The astronomers then used the WFC3 observations to map the cluster’s mass by calculating how much cluster mass was needed to produce the gravitational arc. Chandra data, which revealed the cluster’s brightness in X-rays, also was used to measure the cluster’s mass.

“The chance of finding such a gigantic cluster so early in the universe was less than 1 percent in the small area we surveyed,” said team member Mark Brodwin of the University of Missouri-Kansas City. “It shares an evolutionary path with some of the most massive clusters we see today, including the Coma Cluster and the recently discovered El Gordo Cluster.”

An analysis of the arc revealed that the lensed object is a star-forming galaxy that existed 10 billion to 13 billion years ago. Gonzalez has considered several possible explanations for the arc, though he remains unconvinced pending further study.

One explanation is that distant galaxy clusters, unlike nearby clusters, have denser concentrations of galaxies at their cores, making them better magnifying glasses. However, even if the distant cores were denser, the added bulk still should not provide enough gravitational muscle to produce the giant arc seen in Gonzalez’s observations, according to a statistical analysis.

Another possibility is that the initial microscopic fluctuations in matter made right after the big bang were different from those predicted by standard cosmological simulations, and therefore produced more massive clusters than expected.

The team’s results will be published in the July 10 issue of The Astrophysical Journal.