What's at the Center of the Galaxy?

Tomoharu Oka (Associate Professor, Department of Physics)

Countless galaxies exist in this universe. These are massive systems of tens to hundreds of billions of stars and vast amounts of gas, bound together by gravity. Galaxies come in various shapes, such as elliptical, disk-shaped, and irregular. And among them are those with extremely bright central cores known as "active galactic nuclei." So, what exactly lies at the center of a galaxy? It makes you curious, doesn't it? But if you think about it calmly, what's the point of knowing such a thing? Just because the center of a galaxy tens of millions of light-years away is unusually bright, will investigating the reason be of any use in our daily lives? No, for the most part, it won't be of any use at all. But once you start thinking about it, it becomes irresistibly intriguing. Why is that? This is due to the "intellectual curiosity" inherent in human beings.

Recent research has revealed that at the center of many galaxies, there appears to be a supermassive black hole with a mass of millions to billions of times that of the Sun. A black hole is a region of spacetime where gravity is so strong that nothing, not even light, can escape. When a star with more than about 20 times the mass of the Sun reaches the end of its evolution and undergoes a supernova explosion, what is left behind is a stellar-mass black hole. The supermassive black holes in galactic nuclei are thought to have grown by repeatedly merging with the numerous stellar-mass black holes created by explosive star formation (starburst) activity near the center. In an active galactic nucleus, gas falling into an unusually massive gravitational source converts its gravitational energy into heat, generating powerful thermal radiation.

The center of our own Milky Way galaxy is recognized as a very small radio source called "Sagittarius A*." Observing the stars around this "Sagittarius A*" orbiting it at tremendous speeds suggests that there is a supermassive black hole here as well. Its mass is about 4 million solar masses. Quite impressive. Surprisingly, however, the nucleus of the Milky Way is very dim for such an impressive black hole. Why is this? The key to solving this seems to lie in the amount of gas falling into the black hole and the efficiency with which gravitational energy is converted into heat.

Our laboratory, established in April 2008, is a very new lab and the only experimental astrophysics laboratory at Keio University. Here, we primarily study the activity at the center of the Milky Way based on spectral line observations using radio telescopes (see photo). Molecules and atoms each emit electromagnetic waves (spectral lines) at specific frequencies. When gas is moving relative to us, the frequency of the spectral lines changes due to the Doppler effect. By measuring the intensity and frequency of these spectral lines, we can determine the physical state, chemical composition, and motion of the gas drifting in space. The latest observational results have uncovered evidence that the currently dim nucleus of the Milky Way was extremely bright in the past (Fig. 1), as well as a rotating gas disk that will serve as fuel for it to shine again in the near future (Fig. 2). In other words, to put it simply, our Milky Way galaxy also has periods when its nucleus shines brightly. We look at other galaxies and make all sorts of assumptions, but perhaps from the perspective of a distant galaxy, they might be thinking something like, "That spiral galaxy's nucleus is strangely bright; it must be a terrible place to live."