NASA's Hubble Space Telescope has broken
the distance limit for galaxies and uncovered a primordial
population of compact and ultra-blue galaxies that have
never been seen before.
The deeper Hubble looks into space, the farther back in time
it looks, because light takes billions of years to cross the
observable universe. This makes Hubble a powerful "time
machine" that allows astronomers to see galaxies as they
were 13 billion years ago, just 600 million to 800 million
years after the Big Bang.
The data from Hubble's new infrared camera, the Wide Field
Camera 3 (WFC3), on the Ultra Deep Field (taken in August
2009) have been analyzed by no less than five international
teams of astronomers. A total of 15 papers have been
submitted to date by astronomers worldwide. Some of these
early results are being presented by various team members on
Jan. 6, 2010, at the 215th meeting of the American
Astronomical Society in Washington, D.C.
"With the rejuvenated Hubble and its new instruments, we are
now entering unchartered territory that is ripe for new
discoveries," says Garth Illingworth of the University of
California, Santa Cruz, leader of the survey team that was
awarded the time to take the new WFC3 infrared data on the
Hubble Ultra Deep Field (imaged in visible light by the
Advanced Camera for Surveys in 2004). "The deepest-ever
near-infrared view of the universe — the HUDF09 image — has
now been combined with the deepest-ever optical image — the
original HUDF (taken in 2004) — to push back the frontiers
of the searches for the first galaxies and to explore their
nature," Illingworth says.
Rychard Bouwens of the University of California, Santa Cruz,
a member of Illingworth's team and leader of a paper on the
striking properties of these galaxies, says that, "the
faintest galaxies are now showing signs of linkage to their
origins from the first stars. They are so blue that they
must be extremely deficient in heavy elements, thus
representing a population that has nearly primordial
characteristics."
James Dunlop of the University of Edinburgh, agrees. "These
galaxies could have roots stretching into an earlier
population of stars. There must be a substantial component
of galaxies beyond Hubble's detection limit."
Three teams worked hard to find these new galaxies and did
so in a burst of papers immediately after the data were
released in September, soon followed by a fourth team, and
later a fifth team. The existence of these newly found
galaxies pushes back the time when galaxies began to form to
before 500-600 million years after the Big Bang. This is
good news for astronomers building the much more powerful
James Webb Space Telescope (planned for launch in 2014),
which will allow astronomers to study the detailed nature of
primordial galaxies and discover many more even farther
away. There should be a lot for Webb to hunt for.
The deep observations also demonstrate the progressive
buildup of galaxies and provide further support for the
hierarchical model of galaxy assembly where small objects
accrete mass, or merge, to form bigger objects over a smooth
and steady but dramatic process of collision and
agglomeration. It's like streams merging into tributaries
and then into a bay.
These galaxies are as small as 1/20th the Milky Way's
diameter," reports Pascal Oesch of the Swiss Federal
Institute of Technology in Zurich. "Yet they are the very
building blocks from which the great galaxies of today, like
our own Milky Way, ultimately formed," explains Marcella
Carollo, also of the Swiss Federal Institute of Technology
in Zurich. Oesch and Carollo are members of Illingworth's
team.
These newly found objects are crucial to understanding the
evolutionary link between the birth of the first stars, the
formation of the first galaxies, and the sequence of
evolutionary events that resulted in the assembly of our
Milky Way and the other "mature" elliptical and majestic
spiral galaxies in today's universe.
The HUDF09 team also combined the new Hubble data with
observations from NASA's Spitzer Space Telescope to estimate
the ages and masses of these primordial galaxies. "The
masses are just 1 percent of those of the Milky Way,"
explains team member Ivo Labbe of the Carnegie Institute of
Washington, leader of two papers on the data from the
combined NASA Great Observatories. He further noted that "to
our surprise, the results show that these galaxies at 700
million years after the Big Bang must have started forming
stars hundreds of millions of years earlier, pushing back
the time of the earliest star formation in the universe."
The results are gleaned from the HUDF09 observations, which
are deep enough at near-infrared wavelengths to reveal
galaxies at redshifts from z=7 to beyond redshift z=8. (The
redshift value z is a measure of the stretching of the
wavelength or "reddening" of starlight due to the expansion
of space.) The clear detection of galaxies between z=7 and
z=8.5 corresponds to "look-back times" of approximately 12.9
billion years to 13.1 billion years ago.
"This is about as far as we can go to do detailed science
with the new HUDF09 image. This shows just how much the
James Webb Space Telescope (JWST) is needed to unearth the
secrets of the first galaxies," says Illingworth. The
challenge is that spectroscopy is needed to provide
definitive redshift values, but the objects are too faint
for spectroscopic observations (until JWST is launched).
Therefore, the redshifts are inferred by the galaxies'
apparent colors through a now very well-established
technique.
The teams are finding that the number of galaxies per unit
of volume of space drops off smoothly with increasing
distance, and the HUDF09 team has also found that the
galaxies become surprisingly blue intrinsically. The
ultra-blue galaxies are extreme examples of objects that
appear so blue because they may be deficient in heavier
elements, and as a result, quite free of the dust that
reddens light through scattering.
A longstanding problem with these findings is that it still
appears that these early galaxies did not put out enough
radiation to "reionize" the early universe by stripping
electrons off the neutral hydrogen that cooled after the Big
Bang. This "reionization" event occurred between about 400
million and 900 million years after the Big Bang, but
astronomers still don't know which sources of light caused
it to happen. These new galaxies are being seen right in
this important epoch in the evolution of the universe.
Perhaps the density of very faint galaxies below the current
detection limit is so high that there may be enough of them
to support reionization. Or there was an earlier wave of
galaxy formation that decayed and then was "rebooted" by a
second wave of galaxy formation. Or, possibly the early
galaxies were extraordinarily efficient at reionizing the
universe.
Due to these uncertainties it is not clear what type of
object or evolutionary process did the "heavy lifting" by
ionizing the young universe. The calculations remain rather
uncertain, and so galaxies may do more than currently
expected, or astronomers may need to invoke other phenomena
such as mini-quasars (active supermassive black holes in the
cores of galaxies) — current estimates suggest however that
quasars are even less likely than galaxies to be the cause
of reionization. This is an enigma that still challenges
astronomers and the very best telescopes.
"As we look back into the epoch of the first galaxies in the
universe, from a redshift of 6 to a redshift of 8 and
possibly beyond, these new observations indicate that we are
likely seeing the end of reionization, and perhaps even into
the reionization era, which is the last major phase
transition of the gas in the universe," says Rogier
Windhorst of Arizona State University, leader of one of the
other teams that analyzed the WFC3 data. "Though the exact
interpretation of these new results remains under debate,
these new WFC3 data may provide an exciting new view of how
galaxy formation proceeded during and at the end of the
reionization era."
Hubble's WFC3/IR camera was able to make deep exposures to
uncover new galaxies at roughly 40 times greater efficiency
than its earlier infrared camera that was installed in 1997.
The WFC3/IR brought new infrared technology to Hubble and
accomplished in four days of observing what would have
previously taken almost half a year for Hubble to do.
Ray Villard -
STScI
Image credits:
NASA,
ESA, G. Illingworth and R. Bouwens (University of
California, Santa Cruz),
and the HUDF09 Team
For more about the Ultra
Deep Field, visit the
Hubble Website. |
