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TOP > Report & Column > The Forefront of Space Science > 2004 > The Origin of the X-ray Background Now Unraveled

The Forefront of Space Science

Development of Next Generation X-band Digital Transponder
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Recent observations reveal that supermassive black holes, with mass of 1 million to 1 billiontimes solar mass, hide in almost every galaxy in the local universe. In our galaxy, the Milky Way, for example, there is a black hole with mass of about 4 million times that of the Sun. How were these black holes, which can rightly be called monsters in the universe, formed? This is one of the most important questions for today's astronomy to answer. Its elucidation is crucial for the understanding of the evolution of whole galaxies in the universe. This article will explain our latest research results concerning the origins of the X-ray background, which is directly linked to this question. Furthermore, I will introduce part of the
growth history of supermassive black holes as suggested by the results of our investigation.

When gases fall into a supermassive black hole, their gravitational energy is converted into radiation with high efficiency and the gas shines brightly. This is called an AGN (Active Galactic Nucleus). The radiated energy is immense, in some cases reaching 1 quadrillion times solar luminosity. As black holes suck in gases, they gain mass of the same amount. In other words, these monsters get fat by eating prey. Once they have become fat, they never lose weight. When the monster eats its prey, the screams (radiation given out by the devoured prey) are observed as an AGN. When there is no food available, the monster is no longer an AGN and looks quiet. (The mature monster never disappears, but hides in the center of the galaxy with bated breath.) In short, AGNs are the growth process, by accretion, of supermassive black holes. Therefore, to understand the cosmological evolution of AGNs is directly linked to the elucidation of the growth process of supermassive black holes.

Significance of Hard X-ray observations

The most efficient method of finding AGNs is to seek high-energy X-ray (hard X-ray) radiation, which has strong penetrating power. Since an AGN emits strong hard X-ray radiation, it is easy to distinguish it from normal galaxies. In visible light, however, it is hard to find a faint AGN because of the contamination by starlight. The biggest problem with using visible light or low-energy X-ray (soft X-ray) radiation is that they have almost no effect on "obscured" AGNs (the most abundant AGN population), which are buried deep in dust or gases.

As described below, the superposition of X-ray radiation from all the AGNs in the universe is observed as "the X-ray background". This means that efforts to quantitatively solve the origins of the X-ray background are to understand the cosmological evolution of AGNs. The most basic observational quantity describing the statistical property of AGNs is the "luminosity function", the representation of the spatial number density of AGNs as a function of luminosity. It takes considerable work to determine the luminosity function of every redshift parameter. We have to resolve the X-ray background into individual AGNs, optically identify them, and determine their redshifts. The development history of X-ray astronomy, from HEAO-1 to GINGA, ASCA, Chandra, XMM-Newton, and NeXT (the mission currently planned by ISAS), is simply a quest for achieving higher sensitivity in the hard X-ray band. The determination of the cosmological evolution of the
hard X-ray luminosity function of AGNs is the goal of X-ray survey astronomy.

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