Cosmic Archeology Uncovers the Universe's Dark Ages (Forwarded)

National Astronomical Observatory of Japan
Tokyo, Japan

September 13, 2006

Cosmic Archeology Uncovers the Universe's Dark Ages

Astronomers using the Subaru telescope in Hawai'i have looked 60 million
years further back in time than any other astronomers, to find the most
distant known galaxy in the universe. In doing so, they are upholding
Subaru's record for finding the most distant and earliest galaxies known.
Their most recent discovery is of a galaxy called I0K-1 that lies so far
away that astronomers are seeing it as it appeared 12.88 billion years
ago.

This discovery, based on observations made by Masanori Iye of the National
Astronomical Observatory of Japan (NAOJ), Kazuaki Ota of the University of
Tokyo, Nobunari Kashikawa of NAOJ, and others indicates that galaxies
existed only 780 million years after the universe came into existence
about 13.66 billion years ago as a hot soup of elementary particles.

To detect the light from this galaxy, the astronomers used Subaru
telescope's Suprime-Cam camera outfitted with a special filter to look for
candidate distant galaxies. They found 41,533 objects, and from those
identified two candidate galaxies for further study using the Faint Object
Camera and Spectrograph (FOCAS) on Subaru. They found that IOK-1, the
brighter of the two, has a redshift of 6.964, confirming its 12.88
billion-light-year distance.

The discovery challenges astronomers to determine exactly what happened
between 780 and 840 million years after the Big Bang. IOK-1 is one of only
two galaxies in the new study that could belong to this distant epoch.
Given the number of galaxies that have been discovered from 840 million
years after the Big Bang, the research team had expected to find as many
as six galaxies at this distance. The comparative rarity of objects like
IOK-1 means that the universe must have changed over the 60 million years
that separate the two epochs.

The most exciting interpretation of what happened is that we are seeing an
event known to astronomers as the reionization of the universe. In this
case, 780 million years after the Big Bang, the universe still had enough
neutral hydrogen to block our view of young galaxies by absorbing the
light produced by their hot young stars. Sixty million years later, there
were enough hot young stars to ionize the remaining neutral hydrogen,
making the universe transparent and allowing us to see their stars.

Another interpretation of the results says that there were fewer big and
bright young galaxies 780 million years after the Big Bang than 60 million
years later. In this case, most of the reionization would have taken place
earlier than 12.88 billion years ago.

No matter which interpretation finally prevails, the discovery signals
that astronomers are now excavating light from the "Dark Ages" of the
universe. This is the epoch when the first generations of stars and
galaxies came into existence, and an epoch which astronomers have not been
able to observe until now.

BACKGROUND INFORMATION:

Archeology of the Early Universe Using Special Filters

Newborn galaxies contain stars with a wide range of masses. Heavier stars
have higher temperatures, and emit ultraviolet radiation that heats and
ionizes nearby gas. As the gas cools it radiates away excess energy so
that it can return to a neutral state. In this process, hydrogen will
always emit light at 121.6 nanometers, called the Lyman-alpha line. Any
galaxy with many hot stars should shine brightly at this wavelength. If
stars form all at once, the brightest stars could produce Lyman-alpha
emission for 10 to 100 million years.

In order to study galaxies like IOK-1 that exist at early times in the
universe, astronomers must search out Lyman-alpha light that is stretched
and redshifted to longer wavelengths as the universe expanded. However, at
wavelengths longer than 700 nanometers, astronomers have to deal with
foreground emissions from OH molecules in Earth's own atmosphere that
interfere with faint emissions from distant objects.

To detect the faint light from distant galaxies, the research team had
been observing at wavelengths where Earth's atmosphere doesn't glow much,
through windows at 711, 816, and 921 nanometers. These windows correspond
to the redshifted Lyman-alpha emission from galaxies with redshifts of
4.8, 5.7, and 6.6, respectively. These numbers indicate how much smaller
the universe was compared to now, and correspond to 1.26 billion years,
1.01 billion years, and 840 million years after the Big Bang. This is like
doing archaeology of the early universe with particular filters allowing
scientists to see into different layers of an excavation.

To obtain their spectacular new results, the team had to develop a filter
sensitive to light with wavelengths only around 973 nanometers, which
corresponds to Lyman alpha emission at a redshift of 7.0. This wavelength
is at the limit of modern CCDs, which lose sensitivity at wavelengths
longer than 1000 nanometers. This one of its kind filter, called the
NB973, uses multilayer coating technology, and took more than two years to
develop. Not only did the filter have to pass light with wavelengths only
around 973 nanometers, but it also had to cover uniformly the entire field
of view of the telescope's prime focus. The team worked with a company,
Asahi Spectra Co.Ltd, to design a prototype filter to use with Subaru's
Faint Object Camera, and then applied that experience to making the filter
for Suprime-Cam.

The Observations

The observations with the NB973 filter took place during the spring of
2005. After more than 15 hours of exposure time, the data obtained reached
a limiting magnitude of 24.9. There were 41,533 objects in this image, but
a comparison with images taken at other wavelengths showed that only two
of the objects were bright only in the NB973 image. The team concluded
that only those two objects could be galaxies at a redshift of 7.0. The
next step was to confirm the identity of the two objects, IOK-1 and IOK-2,
and the team observed them with the Faint Object Camera and Spectrograph
(FOCAS) on the Subaru telescope. After 8.5 hours of exposure time, the
team was able to obtain a spectrum of an emission line from the brighter
of the two objects, IOK-1. Its spectrum showed an asymmetrical profile
that is characteristic of Lyman-alpha emission from a distant galaxy. The
emission line was centered at a wavelength of 968.2 nanometers (redshift
6.964), corresponding to a distance of 12.88 billion light years and time
of 780 million years after the Big Bang.

The Identity of the Second Candidate Galaxy

Three hours of observation time did not yield any conclusive results to
determine the nature of IOK-2. The research team has since obtained more
data that is now being analyzed. It is possible that IOK-2 may be another
distant galaxy, or it could be an object with variable brightness. For
example, a galaxy with a supernova or a black hole actively swallowing
material that just happened to appear bright during the observations with
the NB973 filter. (Observations in the other filters were made one to two
years earlier.)

The Subaru Deep Field

The Subaru telescope is particularly well suited for the search of the
most distant galaxies. Of all the 8- to 10-meter-class telescopes in the
world, it is the only one with the ability to mount a camera at prime
focus. The prime focus, at the top of the telescope tube, has the
advantage of a wide field of view. As a result, Subaru currently dominates
the list of the most distant known galaxies. Many of these are in a region
of the sky in the direction of the constellation Coma Berenices called the
Subaru Deep Field that the research team selected for intense study at
many wavelengths.

The Early History of the Universe and the Formation of the First Galaxies

To put this Subaru accomplishment into context, it is important to review
what we know about the history of the early universe. The universe began
with the Big Bang, which occurred about 13.66 billion years ago in a fiery
chaos of extreme temperature and pressure. Within its first three minutes,
the infant universe rapidly expanded and cooled, producing the nuclei of
light elements such as hydrogen and helium but very few nuclei of heavier
elements. In 380,000 years, things had cooled to a temperature of around
3,000 degrees. At that point, electrons and protons could combine to form
neutral hydrogen.

With electrons now bound to atomic nuclei, light could travel through
space without being scattered by electrons. We can actually detect the
light that permeated the universe back then. However, due to time and
distance, it has been stretched by a factor of 1,000, filling the universe
with radiation we detect as microwaves (called the Cosmic Microwave
Background). The Wilkinson Microwave Anisotropy Probe (WMAP) spacecraft
studied this radiation and its data allowed astronomers to calculate the
age of the universe at about 13.66 billion years. In addition, these data
imply the existence of such things as dark matter and the even more
enigmatic dark energy.

Astronomers think that over the first few hundred million years after the
Big Bang, the universe continued to cool and that the first generation of
stars and galaxies formed in the densest regions of matter and dark
matter. This period is known as the "Dark Ages" of the universe. There are
no direct observations of these events yet, so astronomers are using
computer simulations to tie together theoretical predictions and existing
observational evidence to understand the formation of the first stars and
galaxies.

Once bright stars are born, their ultraviolet radiation can ionize nearby
hydrogen atoms by splitting them back into separate electrons and protons.
At some point, there were enough bright stars to ionize almost all the
neutral hydrogen in the universe. This process is called the reionization
of the universe. The epoch of reionization signals the end of the Dark
Ages of the universe. Today most of the hydrogen in the space between
galaxies is ionized.

Pinpointing the Epoch of Reionization

Astronomers have estimated that reionization occurred sometime between 290
to 910 million years after the birth of the universe. Pinpointing the
beginning and end of the epoch of reionization is one of the important
stepping stones to understanding how the universe evolves, and is an area
of intense study in cosmology and astrophysics.

It appears that as we look farther back in time, galaxies get rarer and
rarer. The number of galaxies with a redshift of 7.0 (which corresponds to
a time about 780 million years after the Big Bang) seems smaller than what
astronomers see at a redshift of 6.6 (which corresponds to a time about
840 million years after the Big Bang). Since the number of known galaxies
at a redshift of 7.0 is still small (only one!) it is difficult to make
robust statistical comparisons. However, it is possible that the decrease
in number of galaxies at higher redshift is due to the presence of neutral
hydrogen absorbing the Lyman-alpha emission from galaxies at higher
redshift. If further research can confirm that the number density of
similar galaxies decreases between a redshift of 6.6 and 7.0, it could
mean that IOK-1 existed during the epoch of the universe's reionization.

These results will be published in the September 14, 2006, edition of
Nature.

The Research Team: Masanori Iye (NAOJ), Kazuaki Ota (University of Tokyo),
Nobunari Kashikawa (NAOJ), Hisanori Furusawa (NAOJ), Tetsuya Hashimoto
(University of Tokyo), Takashi Hattori (NAOJ), Yuichi Matsuda (Kyoto
University), Tomoki Morokuma (University of Tokyo), Masami Ouchi (Space
Telescope Science Institute), Kazuhiro Shimasaku (University of Tokyo)

Note: Throughout this article redshifts were converted to distances and
ages using cosmological parameters of H0=71km/s/Mpc, (Omega)=0.27, and
(Lambda)=0.73 to maintain consistency with previous releases. The research
paper submitted to Nature reports slightly different numbers based on
cosmological parameters of H0=70km/s/Mpc, (Omega)=0.3, and (Lambda)=0.7.

[NOTE: Images supporting this release are available at
http://www.subarutelescope.org/Press.../13/index.html ]