Hubble finds hundreds of
young galaxies in the early Universe
22 September 2006
Astronomers analysing two of the deepest views of the cosmos made
with the NASA/ESA Hubble Space Telescope have uncovered a gold mine
of galaxies, more than 500, that existed less than a thousand
million years after the Big Bang.
These galaxies thrived when the cosmos was less than 7 percent of
its present age (13.7 thousand million years). This sample
represents the most comprehensive compilation of galaxies in the
early Universe, researchers said.
The discovery is scientifically invaluable for understanding the
origin of galaxies, considering that just a decade ago early galaxy
formation was largely uncharted territory.
Astronomers had never seen a galaxy that existed when the Universe
was a thousand million years old, so finding 500 in a Hubble survey
is a significant leap forward for cosmologists.
The galaxies unveiled by Hubble are smaller than today's giant
galaxies and very bluish in colour, indicating they are ablaze with
star birth. The images appear red because of the galaxies'
tremendous distance from Earth. The blue light from their young
stars took nearly 13 thousand million years to arrive at Earth.
During the journey, the blue light was shifted to red light due to
the expansion of space.
"Finding so many of these dwarf galaxies, but so few bright ones,
is evidence of galaxies building up from small pieces - merging
together as predicted by the hierarchical theory of galaxy
formation," said astronomer Rychard Bouwens of the University of
California, Santa Cruz, USA who led the Hubble study.
Bouwens and his team spied these galaxies in an analysis of the
Hubble Ultra Deep Field (HUDF), completed in 2004, and the Great
Observatories Origins Deep Survey (GOODS), made in 2003.
The findings also show that these dwarf galaxies were producing
stars at a furious rate, about ten times faster than is happening
now in nearby galaxies. Astronomers have long debated whether the
hottest stars in early star-forming galaxies, such as those in this
study, may have provided enough radiation to reheat the cold
hydrogen gas that existed between galaxies in the early Universe.
The gas had been cooling since the Big Bang.
"Seeing all of these starburst galaxies provides evidence that
there were enough galaxies one thousand million years after the Big
Bang to finish reheating the Universe," explained team member Garth
Illingworth of the University of California, Santa Cruz. "It
highlights a period of fundamental change in the Universe, and we
are seeing the galaxy population that brought about that
change."
In terms of human lifetimes, cosmic events happen very slowly. The
evolution of galaxies and stars, for example, occurs over thousands
of millions of years. Astronomers, therefore, rarely witness
dramatic, relatively brief transitions that changed the Universe.
One such event in the Universe is 'reheating'.
The reheating, driven by the galaxies' ultraviolet starlight,
transformed the gas between galaxies from a cold, dark hydrogen
soup, to a hot, transparent plasma over only a few hundred million
years. With Hubble's help, astronomers are now beginning to see the
kinds of galaxies that brought about the reheating.
Just a few years ago, astronomers did not have the technology to
hunt for faraway galaxies in large numbers. The installation of the
Advanced Camera for Surveys (ACS) aboard the Hubble Space Telescope
in 2002 allowed astronomers to probe some of the deepest recesses
of our Universe. Astronomers used the ACS to observe distant
galaxies in the HUDF and GOODS public surveys.
Another major step in the exploration of the Universe's earliest
years will occur if Hubble undergoes its next upgrade with the Wide
Field Planetary Camera 3 (WFC3). The WFC3's infrared sensitivity
will allow it to detect galaxies that are so far away their
starlight has been stretched to infrared wavelengths by the
expanding Universe.
The galaxies uncovered so far promise that many more galaxies at
even greater distances are awaiting discovery by the NASA/ESA/CSA
James Webb Space Telescope (JWST), scheduled to launch in
2013.
Co-author Marijn Franx, member of the ESA JWST NIRSPEC science
team, explains: "The JWST will be able to see even further back
into the early Universe, and glimpse the first objects that formed.
ESAs NIRSPEC instrument, can even measure the exact distances of
these objects."
