Research Topics


Our group strives to uncover the basic physics of star formation, planetary systems, active galactic nuclei, the birth of stars and galaxies of the early universe, the characteristics of the light they emit, formation and evolution of galaxies and black holes. We tackle many of the problems of modern astrophysics by solving equations of relativistic radiation hydrodynamics, radiation transport, and gravitational dynamics, which faithfully capture the interaction of light with matter and the complex gravitational interactions, so crucial in the study of the formation and evolution of stars, galaxies, and black holes. To find solutions to the complicated system of equations that describe multi-component celestial bodies and turbulent flow, we conduct large-scale numerical simulations, using, amongst others, taylor-made supercomputers at the Center for Computational Sciences.


Cosmological Galaxy Formation

The remarkable recent observation of large ground-based telescopes and the Hubble Space Telescope, sugget that the birth of galaxies occurred at around one billion years since the Big Bang. On the other hand, the cosmic background radiation, which carries information about the universe when it was only 100,000 years old, indicates that the primordial universe was very uniform. One of the biggest problems of astrophysics is then: how did galaxies form out of this highly uniform matter distribution in the universe.

It is believed that a mysterious substance called dark matter is involved. Its existence is inferred from the dynamics of the stars and galaxies, but its identity is unknown.

It is known from spectral observations of quasars, that the universe, after becoming neutral around 100,000 after the Big Bang, was ionized again at some point. The re-ionization of the universe had a significant impact on the birth of the first stars and galaxies affecting gravitational instability and the thermal evolution of the primordial gas. We have investigated these phenomena in detail by assuming a universe dominated by dark matter, solving the radiative transfer equations, the fluid equations and chemistry relevant for the evolution of the primordial clouds and incorporating the re-ionization field. Through these detailed simualations, we seek to determine the identity of dark matter and understand the birth of galaxies throughout the history of the universe.

Formation of Stellar and Planetary Systems

It is currently believed that stars and planetary systems, such as the solar system, formed in a protoplanetary disk surrounding the central star. There have been various theoretical studies to support this, such as those we have been conducting in our group. Did you come to planet formation how microscopic dust (1) protoplanetary disk evolves. In order to understand exactly from observations how the birth of (2) Stars, and radiation transport simulation that takes into account exactly the transport process of radiation coming propagated from such a situation, the protostar T Tauri that is based on it phenomenological model of the type star.

Active Galactic Nuclei

From a small area of one ten-thousandth or less of the galaxy radius, active galactic nuclei, e.g. quasars and Seyfert, release energy of more than 100 times the total radiation of the galaxy. It is presumed that the source of this enormous energy is gas accretion onto a massive black hole, supporting the idea that a supermassive black hole exist at the centers of many if not all galaxies. However, it is still a mystery as to how such a supermassive black hole can form in the course of cosmic history. To address this problem, we have studied the evolution of black holes and angular momentum transport due to the interaction of matter and background radiation of the early universe. It is also proposed that the radiation from active galactic nuclei can drive strong winds by the radiation pressure, and gas flows into the black hole at the galactic center can occur through the a "radiative avalanche".

Radiative Transfer and Radiation Hydrodynamics

The objective of computational astrophysics, is to analyze through numerical simulations complex non-linear phenomena involved in star-, planet- and galaxy formation, and to elucidate the fundamental physical mechanism at work. Three physical processes are generally responsible in the formation of astronomical objects: gravity, hydrodynamics, and radiation. In the past, cosmological fluid dynamics mainly only took into account gas motions and gravity. The radiation has often been ignored, primarily because calculation costs become unmanagable and the physics becomes complicated. However, it has now become possible to treat radiative transport in three-dimensional space. We do so using new supercomputer hosted at the University of Tsukuba, Center for Computational Sciences. We have developed new computational methods for multi-dimensional radiation hydrodynamics, opening up the field of previously unexplored astrophysical phenomena.