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BoRG 58

In a random sky survey made in near-infrared light, Hubble spied five tiny galaxies clustered together 13.1 billion light-years away. They are among the brightest galaxies at that epoch and very young, existing just 600 million years after the universe's birth in the big bang. Galaxy clusters are the largest structures in the universe, comprising hundreds to thousands of galaxies bound together by gravity. The developing cluster, or protocluster, seen as it looked 13 billion years ago, presumably has grown into one of today's massive "galactic cities", comparable to the nearby Virgo cluster of more than 2,000 galaxies. "These galaxies formed during the earliest stages of galaxy assembly, when galaxies had just started to cluster together," said Michele Trenti of the University of Colorado at Boulder and the Institute of Astronomy at the University of Cambridge in the United Kingdom. "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."

Trenti presented the results today at the American Astronomical Society meeting in Austin, Texas. The study will be published in an upcoming issue of The Astrophysical Journal. Most galaxies in the universe reside in groups and clusters, and astronomers have probed many mature galactic cities in detail as far 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. "We need to look in many different areas because the odds of finding something this rare are very small," said Trenti, who used Hubble's sharp-eyed Wide Field Camera 3 (WFC3) to pinpoint the cluster galaxies. "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." Because distant, fledgling clusters are so dim, the team hunted for the systems' brightest galaxies. These brilliant light bulbs act as billboards, advertising cluster construction zones.

Galaxies at early epochs don't live alone. From simulations, the astronomers expect galaxies to be clustered together. Because brightness correlates with mass, the most luminous galaxies pinpoint the location of developing clusters. The galaxies live in deep wells of dark matter. An invisible form of matter, dark matter makes up the underlying gravitational scaffolding for galaxy construction. The team expects many fainter galaxies that were not seen in these observations to inhabit the same neighborhood. 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 large amounts of gas through mergers with other galaxies. The team's simulations show that the galaxies will eventually merge and form the brightest central galaxy in the cluster, a giant elliptical similar to the Virgo Cluster's M87.

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. Hubble looked in near-infrared light because ultraviolet and visible light from these extremely distant galaxies has been stretched into near-infrared wavelengths by the expansion of space during its long journey. The observations are part of the Brightest of Reionizing Galaxies (BoRG) survey, which uses Hubble's WFC3 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 team estimated the distance to the newly found galaxies based on their colors, but the astronomers plan to follow up with spectroscopic observations, which measure the expansion of space. These observations will help the astronomers precisely calculate the cluster's distance and also will yield the velocities of the galaxies, which will show whether they are gravitationally bound to each other. Spectroscopic observations made last year on another faraway galaxy cluster confirmed its distance of 12.6 billion light-years from Earth. A group of astronomers, led by Peter L. Capak of NASA's Spitzer Science Center at the California Institute of Technology in Pasadena, discovered the cluster using a variety of telescopes, including Hubble. Without spectroscopic observations, it's not clear whether the observed galaxies in Trenti's study are gravitationally bound yet. The average distance between them is likely comparable to that of the galaxies in the Local Group, consisting of two large spiral galaxies, the Milky Way and Andromeda, and a few dozen small dwarf galaxies.

Credit: NASA / ESA / M. Trenti (University of Colorado, Boulder, and University of Cambridge, UK), L. Bradley (STScI), and the BoRG team

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