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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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Entire figure of AGN evolution now revealed

Our research was making great strides to nearing its goal. In addition to the ASCA sample by the ALSS, the AMSS, and deep surveys performed with the SIS detector, we combined samples of the brighter flux side acquired by the HEAO-1 satellite and those of the fainter side acquired by the Chandra satellite, and created a hard X-ray selected sample with an extremely high degree of identification completeness. All that was left to do was to compute the luminosity function! But it was not that easy. We spent six months of trial and error to establish an analysis method that would completely eliminate selection bias.

Figure 2.
Fig. 2 The hard X-ray AGN luminosity function, in which the comoving
spatial number density is given as a function of hard X-ray luminosity.
Each redshift parameter range is indicated by a different mark.

Finally, we unraveled the cosmological evolution of AGN luminosity function including "obscured" AGNs for the first time in the world. In addition, the results presented the first quantitative answer to the origin of the major part of the X-ray background. To get to this point, I have spent 10 years and mankind 40 years, but the answer can be described in a very simple figure. Fig. 2 shows the luminosity function. Fig. 3 shows the same result when the spatial number density of AGNs is plotted as a function of redshift parameter (z). Quasars (high-luminosity AGNs) have the peak around z=2. Meanwhile, the Seyfert Galaxies (intermediate luminosity AGNs) have peak around z=0.7, indicating that they were formed more recently. It is interesting that the findings conflict with the "larger is later" theory of the structure formation of the universe. We think that our results can be explained from the aspect of star formation activity of the host galaxy. Global research trends are shifting away from the evolution of AGN itself to study the relevance of AGNs to the formation process of the host galaxies.

Figure 3.
Fig. 3 Redshift dependence of the AGN comoving spatial number density.
Top(black): low-luminosity AGNs; Middle(red): intermediate-luminosity AGNs; Bottom(blue): high-luminosity AGNs.

As mentioned earlier, AGN evolution is directly linked to the formation process of supermassive black holes. By assuming an appropriate radiative efficiency, we can deduce the mass accretion rate (i.e., amount of prey eaten per unit time) from the luminosity. Further, we can calculate how the total mass (i.e., total amount of eaten prey) of black holes per unit volume in the universe has increased as a function of cosmic time. Fig. 4 is the "black hole growth curve" obtained by such an approach. We find that the present mass density of supermassive black holes calculated by the method meets well that estimated by another method based on the demography of nearby galaxies. A recent, more precise calculation shows that the mass function of black holes is fully explained as well. Is this relation true in a distant universe? Was star formation complete by the time when an AGN activity is triggered? Our challenge continues, with the stimulation of our rivals around the world.

Figure 4.
Fig. 4 The growth curve of supermassive black holes.
The above curve corresponds to the case when the contribution of Compton-thick AGNs
(i.e.,those with an absorption column density exceeding one Compton-scatteringopacity) is included.
Dotted lines indicate extrapolation.


This achievement indicates the possibility that we can open new scientific frontiers by effort and teamwork. Also, the result proves the greatness of the ASCA satellite, which was launched with such fortuitous timing, and the importance of collaborative study on other wavelengths. I would like to express my deep appreciation to the joint researchers involved in the ASCA survey and those who participated in the satellite program.

(Yoshihiro Ueda)

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