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Subaru Telescope Detects the Most Distant Galaxy Yet
and Expects Many More

March 19, 2003

Part 1 of 2

Subaru telescope has found a galaxy 12.8 billion light years away (a
redshift of 6.58; see note 1), the most distant galaxy ever observed.
This discovery is the first result from the Subaru Deep Field
Project, a research project of the Subaru Telescope of the National
Astronomical Observatory of Japan which operates the Subaru
telescope. The Subaru Deep Field (SDF) project team found
approximately 70 distant galaxy candidates by attaching a special
filter designed to detect galaxies around 13 billion light years away
on a camera with a wide field of view. Follow-up observations with a
spectrograph confirmed that two out of nine of the candidates are in
fact distant galaxies. One of these is the most distant galaxy ever
observed. This discovery raises the expectation that the project will
be able to find a large number of distant galaxies that will help
unravel the early history of the universe in a statistically
meaningful manner.

The SDF project is an observatory project of the National
Astronomical Observatory of Japan designed to showcase the abilities
of Subaru telescope and to resolve fundamental astronomical questions
that are difficult to address through Subaru's regular time
allocation system. Most research programs on Subaru telescope are
selected through a competitive time allocation process called Open
Use, which is open to all astronomers but allows a maximum of only
three observing nights every six months. By pooling together
observing nights reserved for the observatory and astronomers that
contributed to the establishment of Subaru Telescope, an observatory
project can address questions that require greater telescope
resources than the typical research proposal. The SDF project's main
goal is to detect a large number of the most distant galaxies
detectable and to understand their properties and their impact on the
evolution of the universe. The speed of light is the fundamental
limit to how fast information can travel (see note 2). When we detect
light from a galaxy 13 billion light years away, that means we are
seeing the galaxy as it was 13 billion years ago. Looking for ever
more distant galaxies means looking at galaxies at earlier and
earlier times in the universe. 

The SDF observations took advantage of the fact that light from
distant galaxies have a characteristic wavelength and shape.
Astronomers think that the earliest galaxies rapidly formed stars
from hydrogen, the dominant form of matter in the universe. The light
from these stars would have excited any hydrogen remaining around
them to higher energy states and even ionize it. When excited
hydrogen returns to lower energy states, it emits light at several
distinct wavelengths. However, most of this light would escape the
young galaxy as an emission line at 122 nanometers because "bluer"
light with shorter wavelengths and higher energy can re-excite other
hydrogen atoms. Since the universe is expanding, the farther away a
galaxy is from us, the faster it is moving away from us. Because of
this movement, light from distant galaxies are doppler shifted to
longer, or redder wavelengths, and this emission line is "redshifted"
to a longer wavelength that is characteristic of the galaxy's
distance and the galaxy itself appears redder. As the light travels
the long distance from its origin to Earth, light at the higher
energy side, or blue side of the emission line, can be absorbed by
the neutral hydrogen in intergalactic space. This absorption gives
the emission line a distinctive asymmetrical look. A overall red
appearance and a strong emission line at a particular wavelength with
a particular asymmetrical shape is the signature of a distant new
born galaxy. 

To detect the most distant galaxies ever observed, the SDF team
developed a special filter that only passes light with the narrow
wavelength range of 908 to 938 nanometers. These wavelengths
correspond to the 122 nanometer emission line after travelling a
distance of 13 billion light years. The team installed the special
filter, and two other filters at shorter and longer wavelengths
bracketing the special filter, on Subaru telescope's Suprime-Cam,
Subaru Prime Focus Camera, and carried out an extensive observing
program from April through May 2002. Suprime-Cam has the capability
of imaging an area of the sky as large as the full moon in one
exposure, a unique capability among instruments on 8-m class and
larger telescopes, and is extremely well suited for surveys of very
faint objects over large areas of the sky. By observing an area of
the sky the size of the moon for up to 5.8 hours in each filter, the
team was able to detect over 50,000 objects, including many extremely
faint galaxies. By selecting galaxies that were bright only in the
special filter and preferentially red, the team found 70 candidates
for galaxies at a redshift of 6.6 (or a distance of 13 billion light
years; see figure 1). 

In June 2002, the team used FOCAS, the Faint Object Camera and
Spectrograph on Subaru telescope, to observe 9 of the 70 candidates,
and confirmed the generally red appearance and an emission line with
a distinctive asymmetry in 2 objects (see figure 2), and determined
that their redshifts are 6.58 and 6.54. The light from these galaxies
was emitted 12.8 billion years ago when the universe was only 900
million years old. The previously observed most distant galaxy, with
a redshift of 6.56, was discovered by looking at a large cluster of
galaxies that can amplify light from more distant galaxies with a
gravitational lensing effect. (See our press release from May 2002,
http://www.naoj.org/Latestnews/200205/UH/index.html.)The SDF
observations is the first time multiple galaxies at such a great
distance have been observed, and without the help of gravitational
lensing. The galaxy with a redshift of 6.58 is the most distant
galaxy ever observed.

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