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
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.