Subaru Telescope Detects the Most Distant Galaxy Yet and Expects Many More
March 19, 2003
Object: Subaru Deep Field
Telescope: Subaru Telescope (8.2m effective diameter)
Instruments: Suprime-Cam (Subaru Prime Focus Camera), FOCAS (Faint Object Camera and Spectrograph), and IRCS (Infrared Camera and Spectrograph)
Filter: i'-band (683-854nm), z'-band (834-993nm), and 908-932nm Narrow-band (NB921)
Observation Dates: April - June, 2002
Exposure Time: 4.7 hours (i'), 5.8 hours (z'), and 5.0 hours (NB921)
Image Orientation: North is up, east is left.
Field of View: 27 arcminutes x 34 arcminutes (for SDF image)
Position: R.A. 13h 24.4m, Dec. 27d 29m (Constellation: Coma Berenices)
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/Pressrelease/2002/05/07/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.
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).
The Graduate University for Advanced Studies
National Astronomical Observatory of Japan