About the Cover

Vol. 91 No. 5 (2015)

  Nearly all galaxies in the present universe contain supermassive black holes in their centers, which have 0.1 million to 10 billion times masses of the Sun. It is a big question in modern astronomy how these supermassive black holes evolved in the history of the universe. Key clues are obtained by observations of active galactic nuclei (AGNs), “growing” supermassive black holes that accrete surrounding matter and emit intense electromagnetic radiation. A majority of AGNs are hidden by gas and dust surrounding the black holes, and hence can be most efficiently surveyed by detecting their hard X-ray emission thanks to its high transmission. The integrated X-ray emission from all AGNs in the universe is observed as the “X-ray background” radiation.
  The review paper of this issue (pp.175-192) summarizes the current understanding of the cosmological evolution of supermassive black holes in galactic centers based on Dr. Ueda and his collaborators' pioneering work. The top figure and bottom-left figure show images of X-ray sky, observed with the fourth Japanese X-ray satellite ASCA and ESA's XMM-Newton observatory, in the fields of the ASCA Large Sky Survey and the Subaru/XMM-Newton Deep Survey, respectively. Significant fractions of the X-ray background are resolved into many individual AGNs in the distant universe. By utilizing these hard X-ray selected samples, the space number density of AGNs is determined as a function of redshift (or cosmic time) and luminosity, as plotted in the bottom-right figure. A higher redshift corresponds to a larger distance from us, and hence earlier cosmic time.
  It is revealed that more luminous (hence more massive) black holes grew in an earlier epoch than less luminous ones. This trend, called “downsizing” or “anti-hierarchical” evolution, is apparently contradictory to a naive expectation from standard “hierarchical” (bottom-up) structure-formation theories of the universe, where more massive dark matter haloes formed later in cosmic time. The discovery of this “downsizing” evolution of supermassive black holes strongly impacts our understanding how the present universe has been shaped.