Subaru Telescope Detects the Most Distant Galaxy Yet and Expects Many More
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.
IMAGE CAPTIONS:
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* Larger Image with Arrows (3.7MB)
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* Larger Image without Arrows (3.7MB)
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[Figure 1: (273KB)
http://subarutelescope.org/Latestnews/200303/SDF/fig1.jpg ]
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 field of
view of the images is 10 by 10 arcseconds.
Copyright (c) Subaru Telescope, NAOJ. All rights
reserved.
[Figure 2: (256KB)
http://subarutelescope.org/Latestnews/200303/SDF/fig2E.jpg ]
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.
Copyright (c) Subaru Telescope, NAOJ. All rights
reserved.
Supplementary material
A list of the 12 most distant galaxies discovered as of
March 2003,
http://subarutelescope.org/Latestnews/200303/SDF/rank2E.jpg (86KB)
A star map showing the location of the Subaru Deep Field
with respect to nearby constellations,
http://subarutelescope.org/Latestnews/200303/SDF/StarMap1E.jpg (421KB)
Professional References
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).
