Hopkins astronomers have discovered a high-energy power
source hidden inside a galaxy 660 million light-years from Earth,
providing new evidence supporting a theory that all such "active
galaxies" harbor quasars in their nuclei.
The astronomers were expecting to find the galaxy's power source near its brightest point. The bright spot, they reasoned, probably was a jet of light emanating directly from the energy source.
But to their surprise they learned that the source producing the bright region actually was located thousands of light-years away; it was hidden in the center of a thick doughnut of gas, shining outward like a spotlight and illuminating a wedge of galactic material that extended into the distance.
"It was a really neat project because we were expecting a result significantly different from what we found," said Christina Tremonti, a graduate student in the Department of Physics and Astronomy who has been working on the project since the summer.
Other scientists involved in the research were Hopkins astronomers Alan Uomoto, Holland Ford, Zlatan Tsvetanov and Gerard Kriss, and Robert Antonucci, an astronomer from the University of California at Santa Barbara.
They released their findings Jan. 13, during a meeting of the American Astronomical Society in Toronto, Canada.
The work touches on mysterious objects called quasars, which look like stars but may actually emit more energy than an entire galaxy. A type of spiral galaxy called a Seyfert galaxy appears to contain a power source resembling a quasar in its nucleus. Astronomers use the term active galaxy to describe both quasars and Seyfert galaxies. It is suspected that quasars play a role in the formation of galaxies, and that the same sort of engine, perhaps a black hole, powers both quasars and Seyfert galaxies, causing them to emit huge quantities of energy.
But scientists have been puzzled by the fact that Seyfert galaxies come in two varieties, and it is less clear whether one of those varieties contains the quasar-like engines. One possible explanation is that both types of Seyferts actually are the same, but they face Earth from different angles; in some cases, depending on the angle, the central power source would be blocked from the view of astronomers.
The latest findings support that theory, providing evidence that both types of Seyfert galaxies may harbor the power sources.
The galaxy--actually a pair of colliding galaxies known as Markarian 463--is located in the constellation Bootes just southwest of the bright star Arcturus in the Northern Hemisphere. Using the Hubble Space Telescope's Faint Object Camera, the researchers pinpointed the energy source in the merging galaxies. The camera is equipped with special filters that enable astronomers to observe polarized light--light in which all the waves are moving primarily in the same direction.
In the case of Markarian 463, light from the galaxy's power source is polarized when it bounces off surrounding gas or dust. By studying the polarization direction of that light, astronomers can pinpoint where it came from--the location of the power source.
Like all Seyfert galaxies, Markarian 463 has an unusually bright point of light at its center. But astronomers were surprised to learn that this galaxy's power source was not in the same location as the bright spot.
"The power source is completely blocked from our direct view, even though it lights up material extending at least 12,000 light-years away from it," Tremonti said. "This is important because it may mean that every active galaxy has a quasarlike energy source at its nucleus, even if that nucleus isn't visible in ordinary images."
Markarian 463 is an example of a class 2 Seyfert galaxy. Class 1 Seyferts look distinctly different from the class 2 variety. They are brighter and their optical spectrum is virtually identical to a quasar's, said Uomoto, a research scientist.
But astronomers have theorized that both classes could actually be the same, viewed at different angles from Earth's position in space. The "unified theory of active galactic nuclei" received a boost in 1985, when Antonucci, who was then at the National Radio Astronomy Observatory, and Joseph Miller, from the Lick Observatory, discovered that some class 2 Seyfert galaxies showed a class 1 spectrum when viewed through polarizing filters. The observation can be explained if the class 1 nucleus is being blocked from direct view but its light is reflected off nearby gas or dust clouds.
The fanlike shape of the illuminated region in Markarian 463 suggests that an obscuring "fat torus," or doughnut, girdles the power source and allows light to escape only in a cone perpendicular to astronomers' line of sight, Uomoto said.
In Seyfert 1 galaxies, the theory says, astronomers are looking directly down the bright center, the doughnut hole. But Seyfert 2 galaxies, like Markarian 463, are not facing Earth; they are seen at more of an angle, so that the torus of gas and dust is obscuring the central region.
The gas or dust clouds surrounding the Seyfert 2 nucleus reflect and polarize light, allowing astronomers to detect the power source.
Note: Photos of Markarian 463 are available online at http://www.jhu.edu/news_info/news/home97/jan97/mark463.html
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