June 26, 2001 Using a technique that peeks over obscuring rings of dust and gas and into the hearts of distant galaxies, a researcher has found evidence suggesting that as many as half of the bright, active galaxies known as Seyfert 2 galaxies may have significantly less active central black holes.
Seyfert galaxies are among the most important sources of information astronomers have on the evolution of galaxies, and the supermassive black holes at their centers are increasingly thought to be the "engines" driving galaxy formation, so a change in the activity levels of the central black holes in some Seyfert 2 galaxies could have big implications.
However, Hien D. Tran, the author of the new study, acknowledges that such a change may still be a long way off. He feels his data "strongly suggest" that some central black holes in Seyfert 2 galaxies are less active than previously thought, but admits that many astronomers will debate his interpretation, and that his theory can only be proven through follow-up observations. "This is to be expected for something that questions what has been the standard paradigm for such a long time," says Tran, a research scientist in The Krieger School of Arts and Sciences at The Johns Hopkins University. Tran's study was published in the June 10 issue of Astrophysical Journal Letters.
"I think if you took a survey of astronomers today, most of them would say the signal of the very active supermassive central black hole is in there in those galaxies, you just haven't looked hard enough," says Tim Heckman, a professor of astronomy at Hopkins who has reviewed Tran's paper. "It's going to be hard to prove the negative."
Seyfert galaxies and astronomers have always been a little bit like the proverbial blind men and the elephant. Stuck with extremely limited points-of-view of an elephant, the blind men came away with very different impressions of what it was.
Astronomers aren't blind, but they are limited to observing galaxies from Earth or its vicinity, and for an object as big as a galaxy, that amounts to a single tightly restricted perspective. Studying more than one example of a particular type of galaxy can yield different points of view, but researchers have to be sure the different examples they use are the same type of galaxy.
That's where the distinctive features of a Seyfert galaxy can increase the challenge. The characteristic trait of a Seyfert galaxy is a blisteringly bright fountain of energetic emissions from the central black hole. Astronomers currently believe a torus- or doughnut-shaped cloud of gas and dust surrounds the central black hole in Seyfert galaxies, and that this cloud shapes the fountain by only allowing the intense radiation from the black hole to erupt from the cloud's dust-free central hole.
Seyfert 1 galaxies are oriented so that the opening of the torus points toward Earth, allowing a direct view of activity in the galactic nucleus. However, Seyfert 2 galaxies are oriented so that the opening of the torus is not visible from Earth, and this makes them look different to astronomers.
Astronomers first came to think of Seyfert galaxies as one type of galaxy seen from different views in 1983, when Tran's future mentor, Joseph Miller of the University of California-Santa Cruz and his student Robert Antonucci, now at the University of California-Santa Barbara, proposed what would become known as the unified model of Seyfert galaxies.
Miller and Antonucci based their unified model, which included the concept of a torus of gas and dust surrounding the central black hole, on Seyferts data gathered through a technique known as spectropolarimetry.
"Spectropolarimetry lets us detect how much of the light from a source comes directly from the source, and how much of it is reflected light from other sources that may be hidden," says Tran. "Any time light is reflected, it becomes polarized, and this technique lets us detect that."
Feeling that a definitive test of whether the unified model applied to all Seyfert galaxies was lacking, Tran set out to use spectropolarimetry to conduct a study of a large number of Seyfert 2 galaxies. With funding from NASA, the National Science Foundation, and the Department of Energy, Tran spent seven years either observing directly or analyzing previously acquired data from 50 Seyfert 2 galaxies.
Aaron Barth, a research fellow at the Harvard- Smithsonian Center for Astrophysics, reviewed Tran's study for publication and called it "the largest survey of its kind to date."
In about half of the Seyfert 2 galaxies Tran studied, he could clearly detect reflected light with spectroscopic features similar to those seen in the center of Seyfert 1 galaxies. Although the fountain of radiation from the area around the central black hole in these galaxies points in a direction that makes it hard to see from Earth, enough of the black hole's spectral signature was reflected by material around the galaxy for Tran to detect it.
"The other half of these galaxies, though, don't seem to fit the model," says Tran. "The signature of the central black hole's activity doesn't appear even when you observe the galaxy in polarized light." Tran has tentatively decided to call these "pure S2s," or pure Seyfert 2 galaxies.
To begin checking if anomalous factors in individual galaxies might be obscuring the central black hole's spectroscopic signature, Tran analyzed several other observable characteristics, and once again found that the pure S2s seemed unusual.
"For example, when you plot the luminosity of these galaxies in radio wavelengths versus their flux ratio in the infrared, you can see that the pure S2s appear on average to be cooler and less luminous in radio waves," Tran says. "This correlates well with the idea that pure S2s may have a less active central black hole. Less active central black holes should heat up the surrounding gas and dust less than active black holes." There are still a number of additional possible explanations to consider, though.
"Starbursts, which are regions of intense star formation, may be muddling the picture," says Heckman, who suggests using X-ray observatories to look beyond the gas and dust to get a better picture of the galaxies in question, perhaps providing more clues as to whether they are genuinely different.
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