The supermassive black hole at the center of our Milky Way galaxy appears to be sort of a wimp, surprised astronomers announced today.
With a mass of 2.6 million suns, its perimeter should be ablaze with X-rays created as trillions of tons of ultra-hot compressed gas vanish into its bottomless maw. Instead, eagerly awaited first findings from the recently launched Chandra observatory show that the output "is really puny," said Gordon Garmire of Pennsylvania State University. A hole that hefty should be "a million or even a billion times brighter than what we're seeing. That's a real puzzle. It's going to challenge theorists to explain why it's so faint."
Discovery of this spectacular galactic underachievement was only one of many unprecedented observations from Chandra--a 45-foot-long X-ray observatory placed in orbit tens of thousands of miles above Earth last July--reported at a meeting of the American Astronomical Society here.
Chandra scientists also revealed several heretofore unknown features of both the local and distant cosmos, including the richest field of X-ray star sources ever seen (in the sword of Orion, which contains the nearest massive star-forming region), the innards of a violently exploding "starburst" galaxy, an eerie supernova remnant containing enough oxygen to supply 1,000 solar systems, and an oddball aspect of the Andromeda galaxy, the closest one that resembles ours in size and shape.
Astronomers have long believed that our galaxy, like so many others, hosts a supermassive, superhungry black hole at its core. In general, there are two ways of detecting such an object--one for each of two main kinds of radiation generated when gas, dust and stellar matter are squashed and heated as they pile up before disappearing into the hole.
The heat strips electrons off the infalling atoms, creating a flood of electrically charged particles called a plasma. When those charged particles hit the titanic magnetic fields that surround the black hole, they begin to revolve in a spiral fashion, in effect creating a giant antenna that "broadcasts" in long wavelengths that can be detected by radio telescopes. Previous radio observations had spotted the apparent location of our galaxy's central black hole toward the constellation Sagittarius.
The other way to detect a hole is to measure the X-ray output from the compressed plasma. But Earth is about 26,000 light years from the center of the Milky Way, and X-rays originating there have to penetrate an enormous amount of matter on the way here. So the X-ray signal from the presumptive black hole has been too faint to observe. Scientists have assumed, however, that if it could be seen, it would be substantial.
Now Chandra finally has detected an X-ray source very close to the radio-wave source. But if it is our central black hole, it is pathetically weak.
There are several possible explanations, most involving the environment around the galactic center. For example, infalling gas might pile up, becoming so heated and pressurized that it pushes back away from the hole again, somewhat like the convection pattern seen in a pot of boiling water. That would limit the amount of gas available to produce X-rays.
Something similar seems to be happening in the Andromeda galaxy, said Chandra scientist Stephen S. Murray of the Harvard-Smithsonian Center for Astrophysics. That galaxy ostensibly contains a central black hole of 30 million solar masses, which should heat its surrounding gas to a few million degrees.
But Chandra has now revealed that the temperature is barely a million degrees, and the hole's X-ray output is even weaker than the Milky Way's. "It's not following the predictions of the normal models," Murray said. "We cannot explain what we're seeing."
Claude R. Canizares of the Massachusetts Institute of Technology announced another Chandra first: a detailed image of the remnant from a supernova--the death explosion of a star 15 to 25 times the mass of our sun--in a nearby galaxy called the Small Magellanic Cloud. Because Chandra's spectroscopes enable scientists to distinguish the specific elements that make up an object, Canizares's team was able to determine that the 1,000-year-old ring (with a temperature around 10 million degrees) contains about 10 solar masses of oxygen, along with lesser amounts of neon and magnesium.
Stellar fusion of hydrogen and helium creates all the elements up to the weight of iron; the rest are fabricated in the extreme blast of the supernova. With Chandra's spectral abilities, "we can learn how all chemical elements are formed inside stars," as well as about the "explosion itself," Canizares said.
More dramatic revelations are expected when Chandra accumulates a gallery of images from the boundary of the expanding cosmos, thus looking far backward in time.