There are a number of open questions concerning the formation of galaxies and stellar/planetary systems. The astrophysics group at the Center pursues research on the formation processes of such objects, which requires large scale numerical simulations.

Cosmology

The observations of very distant galaxies show that galaxies formed just one billion years after the Big Bang, which occurred 15 billion years ago. The measurements of the cosmic background radiation have revealed that the early universe was extremely homogeneous. The discrepancy between the smoothness of the cosmic background radiation, and the highly nonlinear structures like galaxies seen today, could be reconciled in terms of dark matter.

Also, it is widely accepted that the event of the reionization of the universe took place at the so-called dark age of the universe, say redshifts z>5, because the intergalactic medium (IGM) is observed to have been highly ionized at redshifts z<5. How the universe was reionized is an issue of great interest as well as significance in the light of the near future observations by dozens of potential facilities which would gradually unveil the dark age, in particular the formation of galaxies and quasars.

Galaxy formation

Galaxy formation is believed to proceed in a `bottom up' manner, starting with the formation of small clumps of gas and stars that then merge hierarchically into giant systems. The baryonic gas loses thermal energy by radiative cooling and falls towards the centers of the new galaxies, while supernovae blow gas out. Any realistic model therefore requires a proper treatment of these processes, but hitherto this has been far from satisfactory. Our simulation follows evolution from the earliest stages of galaxy formation through the period of dynamical relaxation, at which point the resulting galaxy is in its final form. The bubble structures of gas revealed in our simulation (for times of less than 3 x 10⁸ years) resemble closely high-redshift Lyman-alpha emitters. After 10⁹ years, these bodies are dominated by stellar continuum radiation and then resemble the Lyman break galaxies, which are high-redshift star-forming galaxies. After 1.3 x 10¹⁰ years, these galaxies resemble present-day ellipticals.

Active Galactic Nuclei

How the activities of quasars and active galactic nuclei are triggered is a long-standing issue. We consider a novel radiative mechanism due to a circumnuclear starburst. Also, there are recently accumulated a lot of evidences that a massive black hole inhabits the center of a galaxy. The formation of massive black holes is a significant issue in relation to QSO/AGN activities. We pursue the formation of galaxies and massive black holes in dark matter-dominated universes.

Star and Planetary System Formation

Stars are formed by the collapse of interstellar clouds. The material which possesses some amount of angular momentum, cannot fall into the central protostar, but forms instead a disk-like object, called a protoplanetary disk. The protoplanetary disk affects the growth of the protostar, and also creates planets. Thus, to understand the formation of stellar and planetary systems, the process of protoplanetary disk formation must be analyzed in detail.

In the study of the nonlinear phenomena related to the formation of galaxies and stellar/planetary systems, it is quite essential to make use of numerical simulations. The current supercomputers enable us to perform 6-dimensional radiation-hydrodynamics including gravitational, hydrodynamical, and radiative processes. These simulations would result in a significant advance in our understanding of the formation of galaxies and stellar/planetary systems.