Discovery of mysterious far-infrared radiation
The observation result of the radiation was surprising. According to the analysis of observed spectra and spatial uniformity, it is likely that most of cosmic background radiation is one from infrared galaxies. But, there was radiation constituent that cannot be explained by this assumption. In order to describe this issue, in Fig. 3 we show three wavelength data observed by AKARI of the cosmic background radiation, and compare them to the radiation of all galaxies in the universe estimated by theoretical model. This model can be well explained by the observation results by Spitzer Space Telescope (launched in 2003) and the most advanced Herschel Space Telescope (launched in 2009) which observed distant galaxies than AKARI, and other observation data in the past including AKARI. Surprisingly, the brightness of the cosmic background radiation measured by AKARI is up to twice as bright as that of all galaxies. Although it is merely twice, the difference is significant because it is related to the entire energy of the universe.
From where does the radiation come?
What are celestial bodies that emit mysterious far-infrared radiation like? High radiation efficiency is required to cover such large radiation energy by the limited material amount in the whole universe. Looking at Fig. 3 carefully, the peak of the surplus bright constituent is located at shorter wavelength side than that of the whole galaxy radiation and, therefore, is guessed to be radiation from much higher-temperature materials than the galaxies. Besides, if the radiation source is in far distant universe, wavelength is measured to be longer due to the effect of the expansion of the universe. Therefore, the temperature must be much higher at that time. Considering these points, we suppose that the radiation source is not warm dusts heated by stars, but gases and dusts heated to high-temperature when they fall in black holes.
In addition to the brightness of the cosmic background above, AKARI also measured apparent smoothness. I cannot discuss this issue in detail here, but it is concluded that, since the observed cosmic background radiation was very smooth, individual celestial bodies contributing to the smoothness are by far smaller than galaxies. My view is that the cause of the mysterious radiation is not a huge black hole located at the center of galaxy, but relatively small black holes that existed in great number in the early universe.
By the way, apart from our observation stated above, it is revealed that cosmic background radiation in the near-infrared (about several µm in wavelength) that we have pursued for years is also brighter several times than radiation from all the galaxies. As released to press in last month under the title "AKARI detected the light from the first stars" (*2), we believe that ultraviolet from the first-generation stars born in 100 to 300 million years after the start of the universe is observed to be near-infrared cosmic background radiation because the ultraviolet wavelength is extended due to the expansion of the universe. This assumption is based on the spectral shape, degree of spectral uniformity, etc. According to theoretical research, the first-generation stars caused supernova explosion at the end of their short life to leave black holes. This may be origin of the mysterious far-infrared radiationEmy speculation is expanding.
Future research development
As discussed above, we have no direct evidence for the assumption that the source of the mysterious radiation is primeval black holes left by the first stars in the universe. Rather, the origin may be more amazing physical phenomena. Examples of possible candidate include decay of undetected dark matter particle to photon and thermal radiation of Oort Cloud in the solar system, which is thought to be nest of the comets. For elucidation of the mystery, we will take various approaches, not limited to close analysis of the AKARI data.
SPICA, a future large-size infrared astronomical satellite, should identify most of expected primeval infrared galaxies individually with its higher-detection capability by several orders of magnitude than that of AKARI. I am excited to think that "Will the mysterious radiation be still left in the background?" Including the rocket experiment CIBER (Cosmic Infrared Background ExpeRiment), we are aggressively engaged in, to measure cosmic near-infrared background (see ISAS News, No. 338, May 2009 and No. 353, August 2010), we will conduct small but talented sounding rocket experiments. In the future cosmic background radiation exploration mission EXZIT (Exo-Zodiacal Infrared Telescope) following CIBER, planetary explorers such as solar sail will go to deep space to conduct high-accurate observations without influence of the dust of the solar system.
With full range activities of these projects, we must elucidate the mystery that AKARI left. I believe that a significant discovery will wait for us beyond the exploration of the infrared cosmic background radiation.