TOP > Report & Column > The Forefront of Space Science > 2015 > The Night Sky Is Bright!? - About the Mysteries of the Near-infrared Background Radiation -
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Are Limits from Gamma Ray Observation Correct? Here, although the results of direct observation of near-infrared background radiation intensity has been disfavored by the gamma ray observation, is there no problem with the method using gamma rays? When estimating the background radiation intensity from gamma ray, we need to assumeEthe spectrum before the absorption. We assume that it is in the simple shapes like power functions and the spectrum will not suddenly become bright in the TeV (tera- electron-volt, tera: 1012) band. Is this supposition reasonable? With the progress of the galaxy formation theory, the absorption amount by the optical and infrared background light from the galaxy became able to be estimated in detail. With such model, we know the intrinsic gamma ray spectrum of blazar before absorption. However, gamma ray spectra of some blazars such as 1ES 0229+200 is not well explained by standard blazar emission models and we need to think about a new emission mechanism. There have been several reports on such objects and we have not completely understood intrinsic spectra of blazars. Therefore, currently, there is uncertainty in the estimated value of the near-infrared background radiation level from gamma-ray observations. In 2018, the next generation gamma ray telescope Cherenkov Telescope Array (CTA) will start operation. With the high-sensitive observation by CTA, we can make progress in revealing the blazar emission mechanism. Especially, in my prediction, we can investigate the universality of the emission mechanism with the statistical information of blazars obtained by CTA. The day is coming when we are able to definite the optical and infrared background emissions with a high accuracy with gamma rays. Supporting Evidence for the Excess According to Direct Observation And now, although there is uncertainty, the indirect observation technique through gamma rays has been adopted in these 10 years. On the other hand, what about the direct observation of near-infrared background radiation? An important progress on the direct observation during these 10 years was the observations of fluctuation of the sky, namely, the anisotropy (irregularity) of the brightness of sky. According to the infrared astronomy satellites Spitzer and AKARI and the rocket experiment CIBER, in the wavelength range of 1~5μm, excess in the fluctuation of the sky which is hardly explained by galaxies has been found, and it has been the supporting evidence for that there are other components except those come from galaxies veiled in the near-infrared background radiations. Especially, if we look at the spectrum of the fluctuation, we see that it shows the Rayleigh-Jeans Law (the spectral radiance of electromagnetic radiation from a black body) like the stars. This feature is also seen in the spectrum excess. The fluctuation, according to the latest theory, may be originated in stars scattered into the galactic halo field because of the collisions between galaxies. However, since we do not know the amount and distribution of the scattered stars, we are not sure whether it can explain the extra component of the spectrum. The investigation on the stars scattered to the halo field may be an issue for the direct observation in the future. For example, if we investigate the frequency of occurrence of supernova explosion outside a galaxy, we are able to investigate the amount of stars flicked to the halo. Currently we do not understand the origin of the excess of the near-infrared background. It is really a shame for theorists. However, there have been progresses in the indirect observations with gamma rays and the direct observations on the fluctuation of the sky in these 10 years. Especially in the rocket experiments CIBER and CIBER2 aiming at direct observations conducted by Dr. Matsumoto and Matsuuras international research group, we are going to find new factors. By combining these with the observations through gamma ray telescope and conducting researches multilaterally, we would be closer to the mystery of the excess of near-infrared background radiation in the future. In addition, please allow me to use this opportunity to express my gratitude to Dr. Shuji Matsuura (Kwansei Gakuin University) again for his kind advises on this article. (Yoshiyuki Inoue)
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