Jan. 9, 2002 Press Release

Frequently Asked Questions

What is the key issue?

X-ray emission from the Galactic center region of several hundreds of light-years across was discovered about two decades ago. But the origin of this bulge of X-ray emission has remained a mystery ever since. Previous X-ray surveys all had very limited spatial resolution and could barely resolve the emission. The X-ray spectral signature of the emission, if diffuse in origin, indicates the presence of large amounts of very hot gas at temperatures of 100 million degrees or so. However, we don't know how such hot gas might be produced and confined in the center region, as the hot gas tends to expand or to cool very quickly. 

Of course, the center region of our Galaxy is a very unique place. There are a lot of massive stars that have been formed recently. These stars don't live very long. When they die, they produce supernova explosions, which may have heated the gas. But the so-called supernova remnants seen in other parts of the Galaxy are not nearly as hot as what is inferred from the Galactic center region. The magnetic field may also be extremely high in the Galactic center region, which may help to confine the gas. Our Chandra survey is to address such issues and to reveal what is really happening in the region.

What have we learned?

With the high resolution imaging capability of Chandra, our survey has allowed us for the first time to cleanly separate point-like and diffuse X-ray components. Our survey shows that there are hundreds of compact X-ray-emitting sources in the center region, in addition to large amounts of diffuse X-ray-emitting gas. Much of the spectral signature for the very hot gas is surprisingly due to these sources and to the scattered light from such sources. We can draw two conclusions from this discovery. One is that the diffuse gas is not as hot as was thought previously; the gas temperature is probably a factor of about 10 lower. The other conclusion is that there is a new population of X-ray sources that we don't know much about yet. Most likely, these are stellar remnants, such as white dwarfs, neutron stars, and/or black holes.

We further find that the diffuse X-ray emission is associated with distinct interstellar structures observed at radio and mid-infrared wavelengths, suggesting a common origin in recent massive star formation. 

In addition, we find that much of the diffuse X-ray emission is not due to hot gas. A fraction of the emission is clearly due to the reflected light from discrete X-ray sources, such as compact stellar remnants and stellar clusters and possibly from a giant burst of radiation from the central massive black hole in the recent past (a few hundred years ago).

Why is the study important?

Our findings represent an important step toward the understanding the energetic activities in the center region of our Galaxy. This region is around the dynamic center of the Galaxy, which we know contains a massive black hole with a mass of a few million solar masses.  The environment in the region is unique with high gravitational and magnetic fields as well as inflowing mass, feeding the central black hole and active star formation, and outflowing gas, enriching the galactic halo and the rest of the Galaxy. But we still don't have much quantitative information about these phenomena, let alone the detailed knowledge about the physical processes involved. The survey provides the essential information about the X-ray emission from the various high-energy species, which will help us to explore their origins and to study their impacts on the Galactic environment.

Such a study of the Galactic central region is also essential for our understanding of other galaxies. The knowledge we learned from our study will help us to interpret observations of galaxies which show similar phenomena in their center regions. These galaxies are much farther away and therefore are much difficult to study in details. 

Energetic activities in galactic center regions affect the ecosystem of galaxies. You may imagine such activities as gigantic galactic volcano eruptions. The metal enriched hot materials from such an eruption, spreading over a galaxy, can significantly affect not only the "weather" of the galaxy, but also its chemical evolution history. In the extreme cases, which are believed to occur very often in the early universe, the hot gas may escape from a galaxy and pollute the intergalactic environment.