Division of Astrophysics


Masayuki Umemura, Professor, Director of CCS

He graduated from Department of Physics in Hokkaido University, and obtained the doctoral degree in 1987 on “The Formation of Stellar Shells and X-Ray Coronae around Elliptical Galaxies”. Then, he became a postdoctoral fellow in Yukawa Institute of Theoretical Physics in Kyoto University. In 1988, he was hired as an assistant professor in NAOJ. In 1993, he moved to CCS in University of Tsukuba as an associate professor, and serves as a full professor since 2002.

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The Universe is thought to be born 13.7billion years ago with the Big Bang. Hydrogen atoms formed in the universe 0.38 million years after the Big bang (“cosmic recombination epoch”). There were neither stars nor galaxies at the recombination epoch, but there were density fluctuations with the contrast of 1/100000. On the other hand, a number of newly-born galaxies are discovered by observations of the universe at 0.1Gyr after the Big bang. The era from 0.38 million years to 0.1Gyr is called “Dark Age”, and enshrouded in mystery. It is believed that, in the dark age, the first generation stars were born and the primordial galaxies formed (Fig. 1). Thereafter, the merger of small galaxies leads to the formation of massive galaxies, and also clusters of galaxies and a large-scale structure formed. For the cosmic structure formation, dark matter plays a key role. In this research group, the formation processes of first stars, galaxies, massive black holes and large-scale structure, by numerical simulations which take into consideration dark matter, atoms, and radiation. To perform intensive numerical simulations, a dedicated high-performance parallel computer “Cosmo-simulator FIRST” has been developed through the collaboration with computer scientists.

Fig.1 History of the Universe

Research topics

Formation of first generation stars

Using three-dimensional radiation hydrodynamic simulations, we explore the formation of secondary Population III stars under radiation hydrodynamic feedback by a preformed massive star in the early universe.Each panel shows the evolution of a collapsing cloud nearby a first luminous star as a function of time (Fig. 2).

Fig.2 Secondary star formation in the early universe

Galaxy collision and merger

Large galaxies such as the Andromeda galaxy are believed to have formed in part from the merger of many less massive galaxies. Here, we have studied the interaction between an accreting small galaxy and the Andromeda galaxy using N-body simulations. Each track along an arrow corresponds to the evolution of the projected stellar mass density of the merging small galaxies with the different binding energy(Fig. 3).

Fig.3 An accreting small galaxy and the Andromeda galaxy.

Formation of clusters of galaxies and a large-scale structure

Formation of galaxy clusters and the large-scale structure in the universe is investigated to understand the origin and the ingredients of the universe in conjunction with particle physics. Numerical simulations of their formation involve gravitational interaction of massive component (dark matter and baryons) and gas dynamics of baryons. The results of simulations are confronted with the actual astronomical observations.

Formation of primordial galaxies

Formation of massive black holes and evolution of active galactic nuclei

Intensive numerical simulations of radiation hydrodynamics

Development of dedicated computers and exploitation of computational technique

Web sites

Theoretical Astrophysics Laboratory

FIRST (Japanese)