By David Shiga
Peering back to an era not long after the birth of the cosmos nearly 14 billion years ago, two teams of astronomers have captured the earliest snapshots yet of the emergence of galaxy clusters, one of the universe’s basic motifs.
“I’m just sort of blown away by this,” comments Marc Davis of the University of California, Berkeley. “If, 20 years ago, you said you were going to detect [galaxies at this distance], they wouldn’t have believed you.”
The two research teams studied galaxies so distant that the light now arriving at telescopes reflects conditions of the galaxies at least 12.7 billion years ago. Rather than being evenly spread through space, these budding galaxies appear to be forming in groups, or protoclusters. That’s consistent with a leading cosmological theory in which the earliest galaxies formed in the densest pockets of primordial material.
One of the teams, led by Massimo Stiavelli of the Space Telescope Science Institute in Baltimore, Md., used the Hubble Space Telescope for its observations. The other, led by Masami Ouchi of the same institute, used the Subaru Telescope on Mauna Kea, Hawaii.
These two studies, plus two others scrutinizing later stages in the early history of the universe, have refined the time line for the development of galaxy clusters. For example, Ouchi’s team determined that the two protoclusters that it examined are forming stars at a prodigious rate.
Meanwhile, images of a protocluster from an era about 2 billion years later reveal a considerable number of mature-looking galaxies that had already formed large populations of stars. A research team led by Charles C. Steidel of the California Institute of Technology in Pasadena, Calif., used the infrared Spitzer Space Telescope to make those observations.
The Caltech group also found evidence that galaxies in protoclusters mature more quickly than do galaxies in open space, referred to as the field. They observed that the protocluster galaxies possessed bigger star populations than galaxies outside the protocluster and seemed to have formed their stars earlier.
“The difference in galaxy properties is important,” says Pieter van Dokkum of Yale University. “[It] suggests that the differences between field and cluster galaxies that we see today were already imprinted at an early time, when the clusters were still in the process of formation.”
A fourth team, led by Christopher R. Mullis of the University of Michigan at Ann Arbor, detected a seemingly mature cluster from 4.6 billion years after the Big Bang. Its major period of star formation appears to be over.
Mullis and his colleagues used the orbiting XMM-Newton X-ray Observatory and the Very Large Telescope in Atacama, Chile. They now are searching for more clusters from that ancient time period.
Ouchi’s team plans follow-up observations to investigate the history of star formation in the protoclusters it discovered. The researchers are examining how such protoclusters might have helped make the universe transparent by ionizing the opaque gas that previously permeated the universe.
The Ouchi study appears in the Feb. 10 Astrophysical Journal Letters. The other reports will appear in future issues of the Astrophysical Journal.
