The Origin of Background Radiation: New Facts
IRTS and COBE observations found that, in the near-infrared region, there are background radiation components that cannot be explained by known celestial bodies. With just background radiation observation, it is difficult to discern their origin. With recent new observation methods, however, there has been some progress in research.
One method is high-energy (TeV-domain) gamma-ray observation. The existence of galaxies emitting gamma rays was already known. Recent observations showed that the spectra of distant gamma-ray objects are sharply absorbed around 1TeV (Fig. 3). It is believed that gamma rays, originally a simple-exponent spectrum (straight line in the upper part of Fig. 3), collide with near-infrared photons of background radiation in space between galaxies, generate sets of electrons and positrons, and are then finally absorbed. In fact, if we apply the spectra on the main figure, the observed gamma-ray spectra can be reproduced well. This result strongly supports the theory that the origin of cosmic near-infrared background radiation is not within the solar or galactic systems, but that it should be studied with a cosmological approach.
The other observation method is the Wilkinson Microwave Anisotropy Probe, WMAP, launched by NASA in 2001. WMAP precisely defined the cosmological parameters based on thorough observation of CMB fluctuation, and also identified CMB polarization for the first time. CMB polarization is caused by scattering of CMB photons with electrons (Thomson scattering) in intergalactic plasma. From this fact, it is possible to identify the volume of ionized gas that lies on the way until CMB photons travel to us.
Plasma was neutralized 400,000 years after the beginning of the universe. Current intergalactic space, however, is in a highly ionized plasma state. When and how the universe was re-ionized was a mystery. According to WMAP, the re-ionization of the universe is older than had been thought. It took place earlier than 300 million years after the beginning of the universe (z 17). Of the various views on the cause of the re-ionization, the most probable is the ultraviolet rays emitted from the first stars (or “population III” stars) in the Universe. Right after neutralization, the only matter in the universe was helium and hydrogen. Unlike present galaxies, there was no cosmic dust or heavy atoms to cool the gases. So, a theoretical presumption is that the gases could hardly contract and, eventually, would have formed huge stars growing up to hundreds of times the solar mass of. These huge stars emitted most of their radiation as ultraviolet rays, which highly ionized the surrounding gases. We believe this caused the re-ionization of the present universe.
Getting back to the previous topic, can the first stars in the universe really explain the cosmic near-infrared background radiation observed by IRTS? According to an Italian group, the spectra of cosmic near-infrared background radiation shown in the main figure can be reproduced with population III stars. The Italian group states that ultraviolet rays from these stars interact with interstellar gases, eventually transferring most of their energy to the hydrogen bright lines (Lyman-rays, 1215). The spectra on the main figure can be explained by overlapping the lines' red-shifted light. A spectra gap at around 1m can be concluded as a sign that the formation of population III stars ended 600 million years after the beginning of the universe (z 9). In other words, we think that the Lyman- rays from the nearest stars in the population III stars are seen red-shifting to around 1m.
It is now possible for us to presume that near-infrared-region background radiation could have been light from the first stars in the universe. But the theory remains qualitative and there are still many problems to discuss. In particular, the large fluctuation of the background radiation identified by IRTS still refuses theoretical explanation. The task for us observers is to produce new data with higher quality for better understanding. Fortunately, Japan is in a position to lead the world in observations of this kind. Japan's first infrared astronomical satellite ASTRO-F, expected to be launched in 2006, can observe slight fluctuations of background radiation with its imaging at a wavelength of 2m. Japan is developing a rocket experiment (CIBER) in cooperation with the U.S.A. and Korea to observe spectra at around 1m and fluctuation at large angles. We also propose to carry out a zodiacal light-free observation on a solar-sail mission loaded with observation instruments. The observation of cosmic near-infrared background radiation started in Japan. I hope our country will continue to lead the world in this field of observation.
March 2005 marks the 10th anniversary of the launch of the SFU. Although the data were no more than what we could acquire a decade ago, I am still filled with pride that we were able to work at the forefront of astronomy. I would like to tender my heartiest gratitude to Prof. Kuriki and those who helped us with SFU, as well as to co-researchers of IRTS.