Coevolution of Massive Black Holes and Galaxies
Over the course of cosmic evolution that began with the Big Bang, various types of galaxies have been created. How were such diversified galaxies formed? And how was the Milky Way Galaxy that contains our earth formed? Looking for the answer, astronomers have been dedicated to extensive theoretical and observational researches.
In the field of theoretical researches, a number of simulation outcomes have been obtained, and some of them can be seen with a 3D view of the 4D2U project at the National Astronomical Observatory of Japan (NAOJ). Among their programs, there is a visualized astronomical N-body simulation showing how the distribution and behavior of dark matter (comprising 1/4 of the total mass of the universe) have changed over the period of 13 billion years from the time when the size of the universe was about 1/40. The 3D view simulation gives us a clue to the understanding of how stars were formed in a uniformly expanding universe just after the Big Bang, and how they evolved into galaxies. According to the widely believed theory, galaxy clusters evolve to a giant elliptical galaxy after repeated galaxy mergers. Based on this theory, the 4D2U project has simulated and visualized the formation process of a giant elliptical galaxy through numerous galaxy mergers in a high density region of more than 1000 galaxies. A galactic merger is an event that is thought to take place over a period of several billion years. There is also a visualized simulation of the moment of galactic collision where two galaxies are merging into one.
Meanwhile, in the field of observational research, various forms of galactic structures have been observed with different types of telescopes. These observed structures (see the figure above) have yet to provide firm evidence to support the theory of giant elliptical galaxy formation as a result of galaxies collision and mergers; however they do not present any contradictory evidence to it.
The moment of two galaxies merging into one has already been captured in an observation (see the left figure). Based on the assumption that every galaxy contains a supermassive black hole in its center, it is likely that a pair of black holes, namely, binary black holes (BBHs) will be detected in the center of a giant elliptical galaxy which was formed as a consequence of a merger of two galaxies.
BBHs are thought to be in orbital motion where one black hole with a larger mass and another with a smaller mass interact with each other. In recent years, many research papers have suggested the presence of BBHs, and there are high expectations for direct detection of two merging black holes.
Black holes are divided into three types according to their mass: 1) "Stellar-mass black hole" which is believed to be formed after a star collapsed at the end of their life cycle; 2) "Supermassive black hole" which exists at the galactic center and has a mass in the range of hundreds of thousands to billions times of solar masses; and 3) "Intermediate-mass black hole" whose mass is larger than stellar black holes and smaller than supermassive black holes. As black hole merger could have important implications in the evolution process of black holes, capturing the moment of black hole collision would be a clue to the understanding of mysterious black hole formation mechanism.
Studying the function of black hole merger in the formation process of supermassive black holes will lead to a further understanding of the function of galaxy merger in the galaxy formation process. Clarifying the BBH formation mechanism will be a key to the mystery of giant galaxy formation mechanism.