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A Heavenly Zoo with Peculiar Animals
Figure 1: The Milky Way (credit & copyright: Serge Brunier, http://antwrp.gsfc.nasa.gov/apod/ap080104.html)
If you visit a zoo, you can see many different kinds of animals and learn their diverse behaviours. As well as a zoo on the earth, there is a "heavenly zoo" where you can see many different kinds of celestial objects and learn their diverse characteristics. That is the center of our Milky way, the " Galactic center ". The Galactic center is not only the place where many stars concentrate at, but also the place which harbours particular environments such as very low temperature gas clouds (where new stars are born), strong magnetic fields and so on. Therefore, the Galactic center is a precious laboratory for astrophysical study.
The most popular beast is the giant "monster" lurking in the Galactic nucleus. That is a very massive black hole, that is, amazingly, several-million times heavier than the sun (figure 1). Curiously, this black hole is unusually quiet in X-rays; perhaps it is under a long sleep now.
It is impossible to see through the Galactic center region in optical wavelength, since the optical lights are blocked by very thick dusts in the Milky Way. Luckily, we can observe the Galactic center in other wave-lengths such as radio waves, infra-red lights and X-rays. Through radio observations, some objects are named as "mouse" or "snake" based on their morphologies; this is really a zoo! The Galactic center is one of the main targets of the Suzaku satellite. Thus, Suzaku spent a large chunk of observation time on the Galactic center region, and some of the data are immediately released to the public. You may look at these data on JUDO (Figure 2). In the following, please let us introduce some of the major achievements from these observations.
- Extremely hot gas filling the central region
In the Galactic center region, there is an extremely hot gas hotter than one-hundred-million degrees Celsius. Some people thought this is due to concentration of hot stars, but Suzaku revealed that such a hot gas is rather uniformly and widely distributed. What an environment!
- Mirrors showing monster's past
At the Galactic center, X-rays are emitted not only from hot gas, but also from very cold gas which is under -200 degree Celsius. This is due to reflection of strong X-rays by the cold clouds. Such reflection nebulae were previously observed by ASCA, and some more samples are added by Suzaku. It is thought that these reflection nebulae reflects past activities of the super-massive black hole located at central nucleus. X-ray emitted from the black hole are now reaching us with a bit of delay due to it's long path being reflected by the cold reflection nebulae. These reflection nebulae have not been discovered except the Galactic center, so we may call these objects as literally peculiar animals.
In addition to what we have mentioned above, many new objects have been discovered and new results are obtained. Please join us and take a look at the Suzaku Galactic center data using JUDO and UDON.
Figure 2: Enlarged image of the Galactic center region made by JUDO. This area has been pointed repeatedly and many X-ray sources are detected.
Hiroshi Murakami (Rikkyo University College of Science Department of Physics)
January 2009