Satoru IGUCHI

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.

In the galaxy formation process, it is believed that smaller galaxies evolve into giant elliptical galaxies as a consequence of numerous collisions and mergers. The giant elliptical galaxy contains in its center two black holes orbiting each other which are called binary black holes (BBHs). How do these BBHs evolve to a supermassive black hole after collisions and mergers? How does gravitational wave radiation (which is believed to be emitted in the process of collision and merger) affect the black hole formation? These questions are yet to be answered, but we can say that the black hole collision is one of the most spectacular natural phenomena in the universe and would have a dramatic implication on the formation of supermassive black holes.

Artist’s image of BBHs

Although there are several theories regarding the black hole collision and merger, they can be interpreted as a series of phenomena divided into five stages: 1) Two black holes are in orbital motion in the center of a giant galaxy. 2) The two black holes interact with nearby stars and gas and come closer to each other. 3) When the two black holes come close to a certain distance, they emit gravitational waves. 4) The two black holes gravitate toward one another, and then collide and merge into one (see the left figure). 5) Finally, there exists a supermassive black hole in the center of a giant elliptical galaxy.

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In 2003, we detected evidence indicating the presence of two black holes from the orbital motion in the center of the radio galaxy 3C66B at 280 million light-years away. This discovery was published in an American scientific journal "Science" (see the left figure.) 3C66B is a supergiant radio galaxy with radio jets that extends 300 thousand light years (see the figure above), which is also famous as a giant elliptical galaxy. In the history of galaxy formation from the birth of the universe, it is believed that smaller galaxies evolve into giant elliptical galaxies as a consequence of numerous mergers. Based on this, we assumed that the center of the giant elliptical radio galaxy 3C66B would contain two black holes just before collision and merger.

In 2010, we first detected a small periodic flux variation in the center of 3C66B on a cycle of about 93 days by detailed monitoring observations over a period of three years with the Nobeyama Millimeter Array (NMA) and the IRAM Plateau de Bure Interferometer (PdBI) which is jointly operated by France and Germany. This discovery was published in an American astrophysical journal "The Astrophysical Journal Letters" (see the figure above.) Furthermore, from the observation results, we found that one of the two black holes with a larger mass has a mass of about one billion times the mass of the Sun and the distance between the two black holes is approximately 0.02 light years (about 1200 times the distance between the Sun and the Earth). Since black holes are thought to emit gravitational waves in the process of collision and merger, it is estimated that these two black holes could collide and merge in 500 years or so.

Some galaxies have a compact central region with high luminosity called "active galactic nucleus (AGN)." An AGN appears extremely bright when observed at optical wavelengths. It is believed that the galactic center contains a supermassive black hole with a mass one million to one billion times the mass of the Sun, and accretion of matter onto a supermassive black hole produces high energy radiation called relativistic jets. Observation results show that some jets are emitted at close to the speed of light. This most dynamic phenomenon in the universe can be observed at all frequencies from the radio, infrared, optical, x-ray, to gamma-ray waves. AGNs have been categorized into several different types according to their luminosity and the level of emission (e.g. seyferts, blazars, and quasars); however, this classification is now being questioned by astronomers who think that they would have the same properties in spite of apparent difference due to the direction of observation, presence of jets, or position of dusts.

By the survey of early-type galaxies with the Hubble Space Telescope (HST), it was discovered that there is a silhouette disk with a size scale of a few hundred parsecs to a few kiloparsecs around an AGN. However, since it is almost impossible to accurately estimate the physical amount of a silhouette disk and obtain its kinematics data at present, there is still little understanding of its basic properties such as the amount of gas contained, and physical and integral properties. We are aiming to clarify the mechanism of disks, rings and torus of molecular gas associated with AGNs through observation of molecular emission/absorption lines of a silhouette disk in early-type galaxies at millimeter/submillimeter wavelengths.

From the results of the multiwavelength observation with the Very Long Baseline Array (VLBA), it was found that the core of NGC4261 is surrounded by an absorption layer with cool plasma (shown as a gap in the left VLBA image). As shown in Figure 3.2, NGC4261 has a dust disk around the core (observed with HST) and emits prominent symmetric jets (7 kpc) from both sides (observed with the Very Large Array (VLA)). The dust disk is inclined at almost the same gradient as the jet axis, and emission lines [NII] λλ6548 and 6584 from the disk are detected. According to the calculation using these emission detecting points and the radial velocity with Kepler’s circular model, the estimated center mass is approximately 500 million times the mass of the Sun. From the observation with VLA and the James Clerk Maxwell Telescope (JCMT), it was discovered that the nucleus of NGC 4261 is surrounded by molecular gas, emitting absorption lines of HI and CO (2-1). Thus, we carried out a program to observe absorption lines of CO (1-0) and CO (2-1) with the Plateau de Bure Interferometer (PdBI) at the IRAM, an international research institute for radio astronomy which is jointly operated by France and Germany. By further analysis of this observation results, we will be able to precisely measure the intensity ratio and optical depth of two absorption lines and study physical parameters (e.g. excitation temperature) of the molecular gas torus around a radio galaxy that contains an AGN.

Also, we successfully obtained the first images of rotating molecular gas associated with the silhouette disk with CO (1-0) emission lines as a result of high-spatial-resolution observations of a silhouette disk in the central region of 3C31/NGC383 (which was first discovered by HST) with the Nobeyama Millimeter Array (NMA) and Nobeyama Radio Observatory (NRO) 45-m Telescope. This achievement was published in "Astrophysical Journal") (see the left figure.)

Now, we are planning to conduct multi-line/multi-transition observations at high spatial resolution in order to systematically study: 1) accretion disk which is thought to exist around an AGN: 2) mechanism of mass accretion on an AGN: 3) relation between accretion disk and AGN activity: 4) radio jet generation mechanism: 5) physical state of gas: 6) gravitation stability of gas disks and rings: and 7) star forming disks.

3C66B is a galaxy that has a silhouette disk (first observed by HST) and binary black holes in its center (first observed by the relative-VLBI). We obtained the first images of its prominent millimeter-wave jet extending 16 kpc with NMA.

The submillimeter VLBI technique at super high resolution is essential for the study of black holes and accretion disks. However, the problem is that far higher sensitivity is required to match such extremely high resolution. One solution for this is to use ALMA as a base station of VLBI. If this method is established, it will be a major turning point in the black hole research because we would have a great chance of obtaining direct images of black holes with this new method. When ALMA starts operation at its full capacity, we will be able to take a more detailed look at the structure of jets of various galaxies at submillimeter and millimeter wavelengths. I expect these new observation outcomes will lead us to systematic understanding of: 1) relation between prominent jets and black holes: 2) relation between accretion disks and jets: and 3) effect of external gas, and a lot more yet to be discovered.