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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Signs of the First Stars Were Found with Direct Observations? Figure 1 shows the latest results of the observations of the cosmic near-infrared background radiations. The content of this article is all summarized in this figure. Obviously, in the near infrared area, the observation data points are scattered. This is why the infrared background radiation is a big problem in astronomy. In fact, the data can be divided into two classes. The unfilled points are the results of directly observing the background radiations while the filled points stand for results integrating the lights from each galaxy, which is the basis element of the universe. We also show the theoretical expectation of the background radiations from the galaxies with a black solid line according to a model of the formation of galaxies. We can say that the galaxy observation results are almost consistent with the theoretical model and we have almost revealed the contributions from the galaxies. On the other hand, the lights we observed directly are several times brighter than those from the galaxies. This indicates that there are missing near-infrared light sources in the universe besides the galaxies and stars we know. 
There have been various discussions on this excess component. As introduced in Matsumoto’s article in 2005, there is one theory saying the first stars, which are the first stars formed in the universe, are the sources of this excess. Suppose the first stars were the sources, the near-infrared background radiations might have given us a lot of important knowledge on first stars. However, there is a big problem in the first star scenario. In the universe when the first stars formed, electrons and protons were bonded together, and it was neutral. Then, the universe was almost ionized by the intense emission from these first stars. In fact, according to the observations of the reionization epoch, the first star scenario has been almost rejected. If there were as many first stars as to explain the near-infrared background excess, it greatly conflicts with the observation of the degree of ionization of universe. As a matter of fact, we have put a bound on the star formation rate of first stars using the latest data of reionization of universe from the cosmic background radiation observation by the Planck satellite and the constraint from the gamma ray observations, which will be mentioned below. The constrained star formation rate is 2 orders of magnitude less to explain the near-infrared background radiations. The first star scenario was rejected and the investigation on the origin of the excess returned to the starting point. By the way, does this excess really exist? In fact, in the direct observation of the near-infrared background radiation, it is a big issue to remove the zodiacal lights. The zodiacal light is the sunlight scattered by the dust in our solar system. That light is much brighter than the near-infrared background radiation. If we failed to estimate the zodiacal light, the excess might appear. Therefore, we have to consider another method to investigate the near-infrared background radiation indirectly instead of direct observation. The excess is disfavored by Gamma Ray Observations You may think that a gamma ray has much higher energy than the infrared light so it should have no relation to the research on near-infrared background radiation. But in fact, gamma ray is playing an important role in the study of the near-infrared background radiation. Especially in the this 10 years, the research of the near-infrared background radiation has made a great progress from gamma ray observations. A high-energy gamma ray (about over 100GeV [giga-electron-volt, giga: 109]) propagating the space is absorbed by the electron-positron pair creation reactions (*2) with the visible or infrared background radiations. The absorbed amount of gamma rays depends on the brightness of the background radiation. Therefore, if it is able to estimate the absorbed amount from the observation of gamma rays from the distant celestial objects, in principle, we can measure the intensity of the background radiation indirectly. From the latter half of the 2000’s, the observation technology of gamma ray telescopes has been improved and it became able to observe gamma rays over 100GeV with a higher sensitivity. Especially there have been many observations on the distant objects called blazars and we became able to get the attenuated gamma-ray spectrum of the blazars (*3). And then, according to the gamma observations of blazars, there have been important progresses in background radiation. In 2006, just 1 year after Prof. Matsumoto’s article, a collaboration team of the gamma ray telescope, H.E.S.S. in Namibia, Africa, reported that the intensity of the near-infrared background radiation was the same as the contribution from the galaxy from distant blazar observations and rejected the results of the direct observations. Other gamma ray telescopes also got similar results with the same method but with more precise constraints. One of the limits according to the high-energy gamma ray telescope MAGIC in the Spanish Canary Islands is shown with the green line in figure 1. By the way, I have also been working on the high-energy radiation mechanism of active galactic nuclei and the cosmic gamma ray background radiation. As I mentioned here, the gamma ray astronomy is important for the near-infrared background radiation. However, as my research on gamma rays went on, I started to see problems of the near-infrared background excess in my own researches and started to work on this field from the latter half of my graduate course. As a result, I was involved in the researches on the cosmic background radiation from infrared light (10-2eV) to the gamma rays (up to 1014eV), covering 16 orders of magnitude in energy. I’m feeling really lucky for this. (*2):Electron-positron pair creation reaction: A phenomenon that electrons (particles) and positrons (antiparticles) being created due to the interactions of lights. (*3):Blazar: When gas falls into the supermassive black holes in the center of galaxies, which is on the order of millions to billions of solar masses, enormous gravitational energy will be released. Due to this, the extragalactic object becomes much brighter than the mother galaxy and we call it the active galaxy nuclei. The blazar is one of the types of the active galaxy nuclei. They have the strong relativistic jet, which is the biggest particle accelerator in the universe, towards us. As the result of the emission from the high-energy particles accelerated at the accelerator and the relativistic beaming effects, a blazar is extremely bright from radio to gamma rays.
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