Hubble Pinpoints Farthest Protocluster of Galaxies Ever Seen

WASHINGTON -- Using NASA's Hubble Space Telescope, astronomers have
uncovered a cluster of galaxies in the initial stages of development.
It is the most distant such grouping ever observed in the early universe.

In a random sky survey made in near-infrared light, Hubble found 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 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, is seen as it looked 13 billion
years ago. Presumably, it 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. 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. "The search is hit and miss. Typically, a region has
nothing, but if we hit the right spot, we can find multiple galaxies."

Hubble's observations demonstrate the progressive buildup of galaxies.
They also 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 collection.

Because the distant, fledgling clusters are so dim, the team hunted
for the systems' brightest galaxies. These galaxies act as
billboards, advertising cluster construction zones. From computer
simulations, the astronomers expect galaxies at early epochs to be
clustered together. Because brightness correlates with mass, the most
luminous galaxies pinpoint the location of developing clusters. These
powerful light beacons live in deep wells of dark matter, an
invisible form of matter that makes up the underlying gravitational
scaffolding for 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 eventually will merge
and form the brightest central galaxy in the cluster, a giant
elliptical similar to the Virgo Cluster's M87.

The observations are part of the Brightest Reionizing Galaxies 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.
Those observations will help astronomers precisely calculate the
cluster's distance and yield the velocities of the galaxies, which
will show whether they are gravitationally bound to each other.

The Hubble Space Telescope is a project of international cooperation
between NASA and the European Space Agency. NASA's Goddard Space
Flight Center in Greenbelt, Md., manages the telescope. The Space
Telescope Science Institute (STScI) in Baltimore conducts Hubble
science operations. STScI is operated for NASA by the Association of
Universities for Research in Astronomy, Inc., in Washington.

For images and more information about Hubble, visit:

http://www.nasa.gov/hubble

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