| 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.
The SDF team expects to find many more
distant galaxies through continued observations. Before
the first stars and galaxies formed, the universe was in
a stage that Astronomers call "the dark ages of the universe".
Determining when the dark ages ended is one of the most
important astronomical questions of our time. Core members
of the team, Keiichi Kodaira from the Graduate University
of Advanced Studies in Japan, Nobunari Kashikawa from the
National Astronomical Observatory of Japan, and Yoshiaki
Taniguchi from Tohoku University hope that by detecting
a statistically significant number of distant galaxies,
they can begin to characterize the galaxies that heralded
the end of the universe's dark ages.
Note 1: The
more distant a galaxy is from us, the faster it is moving
away from us. As a result, light from distant galaxies
are doppler shifted to longer, or redder wavelengths.
This phenomenon, called redshift, is a direct consequence
of the expansion of the universe. The best real life example
of a doppler shift is the change in pitch of the siren
from an emergency vehicle as it passes by. As an ambulance
approaches its siren has a high pitch, or a sound of shorter
wavelength. As it moves away, the siren has a lower pitch
or a sound of a longer wavelength. Astronomers use the
ratio between the shift in wavelength and the original
wavelength of the light from a galaxy to indicate its
distance, and this number is also called redshift. What
distance a redshift corresponds to depends on the overall
structure of the universe. The distances quoted in this
press release are based on recent research indicating
that the universe is 13.7 billion years old.
Note 2: The
speed of light is approximately 300,000 kilometers per
second, the speed required to circle the Earth seven and
a half times in a second. Light travels 10 trillion kilometers,
or one light year, in a year
Note 3: Following
the Big Bang, when the universe came into existence, the
universe was a hot plasma where elementary particles whizzed
about independently. The universe cooled as it expanded,
and about one million years after the Big Bang, the universe
was cool enough for protons and electrons to combine and
form neutral hydrogen atoms. This epoch is called the"dark
ages" of the universe. Astronomers think that when the
first stars and galaxies formed, their light ionized the
neutral hydrogen, and returned the universe to a plasma.
When the first stars formed and the dark ages of the universe
ended is one of the most important astronomical questions
of our time.
 |
Figure 1:
The two newly born galaxies discovered in this study.
Three images of each galaxy are arranged in order
of wavelength from from short (blue) to long (red)
from left to right. The galaxies are bright only
in the middle image corresponding to the narrow-band
filter sensitive to 908-932 nanometers. The galaxies
are not visible in the shorter wavelength i'-band
filter on the left, but they are faintly visible
in the longer wavelength z'-band filter on the right.
The filed of view of the images is 10 by 10 arcseconds.
|
 |
Figure 2:
Spectra of the two newly discovered galaxies. The
122 nanometer emission line of hydrogen has redshifted
to 915-920 nanometers. There is almost no signal
on the shorter wavelength side of the emission line,
while on the long wavelength side there is a shoulder
and some low level signal. |
- A list of the 12 most distant
galaxies discovered as of March 2003.
- A star map showing the
location of the Subaru Deep Field with respect to nearby
constellations.
The information in this press release is
based on a research article to appear in the April 2003
issue of the Publications of the Astronomical Society of
Japan (PASJ Vol. 55, No. 2).
Keiichi Kodaira
The Graduate University for Advanced Studies
Japan
Phone: +81-46-858-1512
email: kodaira_keiichi@soken.ac.jp
Nobunari Kashikawa
National Astronomical Observatory of Japan
Japan
Phone: +81-422-34-3512
email: kashik@zone.mtk.nao.ac.jp
Yoshiaki Taniguchi
Tohoku University
Japon
Phone: +81-22-217-6508
email: tani@terra.astr.tohoku.ac.jp
March 19, 2003 |
