Caution: early galaxy cluster under construction

An astronomy team led by the University of Colorado Boulder using NASA’s Hubble Space Telescope has zeroed in on a wild intergalactic construction project -- a cluster of early galaxies just starting to assemble only 600 million years after the Big Bang.

The five small galaxies clustered together, about 13.1 billion light-years away, are among the brightest galaxies at that epoch and represent the most distant such grouping ever observed in the early universe. One light-year is about 6 trillion miles.

Galaxy clusters are the largest structures in the universe, comprising hundreds to thousands of galaxies bound together by gravity. The developing cluster, or protocluster, presumably will grow into one of today’s massive “galactic cities” comparable to the nearby Virgo cluster, a collection of more than 2,000 galaxies.

“These galaxies formed during the earliest stages of galaxy assembly, when galaxies had just started to cluster together,” says study leader Michele Trenti, a research associate at CU-Boulder’s Center for Astrophysics and Space Astronomy.  “The result confirms our theoretical understanding of the buildup of galaxy clusters. And Hubble is just powerful enough to find the first examples of them at this distance.”

Most galaxies in the universe live in groups and clusters, and astronomers have probed many mature “galactic cities” in detail as far away as 11 billion light-years away. But finding clusters in the early phases of construction has been challenging because they are rare, dim, and widely scattered across the sky.

“Records are always exciting, and this is the earliest and the most distant developing galaxy cluster that has ever been seen,” said CU-Boulder Professor Michael Shull of the astrophysical and planetary sciences department, a member of the observing team. “We have seen individual galaxies this old and far away, but we have not seen groups of them in the construction process before.”

Trenti’s team used the Wide Field Camera 3 to hunt for the elusive catch. “We need to look in many different areas because the odds of finding something this rare are very small,” Trenti said. “It’s like playing a game of Battleship: The search is hit and miss. Typically, a region has nothing, but if we hit the right spot, we can find multiple galaxies.”

The five bright galaxies spotted by Hubble are about one-half to one-tenth the size of our Milky Way, yet are comparable in brightness. The galaxies are bright and massive because they are being fed lots of gas through mergers with other galaxies. Simulations show the galaxies will merge in time to form the brightest central galaxy in the cluster.

The observations demonstrate the progressive buildup of galaxies and provide further support for the hierarchical model of galaxy assembly, in which small objects accrete mass, or merge, to form bigger objects over a smooth and steady but dramatic process of collision and agglomeration. Astronomers have likened the process to streams merging into tributaries, then into rivers and to a bay.

The observations are part of the Brightest of Reionizing Galaxies, or BoRG survey, which is using Hubble’s Wide Field Camera 3 to search for the brightest galaxies around 13 billion years ago, when light from the first stars burned off a fog of cold hydrogen in a process called reionization. The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency.

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