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The Forefront of Space Science

What is Mysterious Far-Infrared Radiation Detected by AKARI?
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Exploration by AKARI of primeval infrared galaxies

Mission of AKARI is not limited to observation of the cosmic background radiation. One purpose for AKARI to create an all-sky map of the universe is to perfectly know all galaxies shining brightly in infrared. Prior to discussion about the mystery of the radiation, I need to mention infrared coming from galaxies, which must be partly contained in the radiation.

One of the most important achievements by IRAS, the first infrared astronomical satellite in Japan (launched in 1983), is the discovery of "infrared galaxies," which are special kind of galaxies shining brightly in infrared rather than in light. Later, their real identity was revealed. They are galaxies in which new stars are born at a furious pace or huge black holes in which massive materials are sucked. Dusts (solid fine particles) existing around there are heated and warmed by intensive ultraviolet from such galaxies or black holes and emit strong infrared. Infrared galaxies are rare in the present universe. But, according to the observations by the ISO satellite (launched in 1995) and etc., it becomes clear that their ratio increases rapidly as going back in time. This means that star formation activity was more active in the ancient universe than at present. Further, extending our speculation, infrared galaxies must exist overflowingly at the early universe. AKARI is produced just to explore such primeval infrared galaxies!

In order to explore infrared galaxies in the early universe, we conducted observation of the area named "AKARI Deep Field South (ADF-S)" in the wavelength of 50 to 180µm using the Far-Infrared Surveyor (FIS) onboard AKARI. The area was selected according to the criterion that dust radiation within our Galaxy impeding observations is most weak. Its dimension is just 1/4,000 of the all-sky map produced by AKARI. Instead, we planned to spare a long time on observation so that we could detect far, dark celestial bodies. As a result, we were able to discover an unprecedentedly large number of infrared galaxies. Fig. 2 shows the image obtained by the observations in the 90µm wavelength band. Dotted white bright spots are all galaxies.

Figure 2
Figure 2. Far-infrared image by AKARI of the AKARI Deep Field South (ADF-S) area in the 90µm wavelength band
Whitish bright spots are individual galaxies. In the background of them, there exists the cosmic background radiation from a numerous number of objects in the early universe.

Cosmic infrared background radiation

Although AKARI boasts high sensitivity, it has a limit in resolution, which is determined by ratio of the telescope size (68 cm) and wavelength (90µm). The telescope is larger in size than human's eye, but the wavelength possible to be measured is longer than visible light. As a result, AKARI's eyesight is almost equivalent to human's one (approx. 1.0). Due to the eyesight, images of primeval infrared galaxies present in great number were overlapped to each other, so we were unable to detect them individually. We once discovered a huge number of galaxies. But, then, we came to notice that numerous celestial bodies are likely to exist far beyond there.

Then we tried a new approach to identify the cosmic background radiation, i.e., bundling the overlapped primeval infrared galaxies into a cluster. This was done by the process to firstly deduct carefully radiation constituents of the solar system and our Galaxy from the entire brightness observed and, then, to scrutinize the remained signals from other extragalaxies. In addition, in order to sort radiation from the far universe, we removed galaxies that were identified individually, even including dark (i.e., far) ones as much as possible. AKARI has high resolution capability 100 times better than that of COBE satellite (launched in 1989), which measured cosmic infrared background. Thus, we were able to remove even the galaxies that were dark by several orders of magnitude. In conclusion, we are confident to say that cosmic background radiation data obtained by AKARI is surely one from distant celestial bodies.

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