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High rate of star births linked to black holes
JOHNS HOPKINS UNIVERSITY NEWS RELEASE
Posted: November 8, 2000

  Ground
This false-color image at visual wavelengths observed from the ground is 200,000 light years across. Credit: Anglo-Australian Observatory and Space Telescope Science Institute
 
The unusually high rates of star births seen in some galaxies may be linked to voracious black holes at the center of those galaxies, according to a new analysis of astronomical data by researchers at The Johns Hopkins University.

The new results suggest that galaxies with rapid rates of star formation and galaxies with active black holes, long considered separate phenomena, are actually links in a single set of evolutionary processes that shape the development of galaxies.

Nancy Levenson, a postdoctoral fellow, and her colleagues in the Department of Physics and Astronomy in the university's Krieger School of Arts and Sciences, report their observations on November 6 in Honolulu at a meeting of the High-Energy Astronomy Division of the American Astronomical Society.

"The process that makes the central black hole in a galaxy grow more massive may help trigger these bursts of significant star formation," Levenson said. "A starburst phase may be a common stage of development for Seyfert galaxies and quasars, two of the brightest objects astronomers have observed in the distant universe."

Seyfert galaxies house active supermassive black holes in a central region no larger than Earth's solar system. As the black hole consumes nearby stars and gas, it unleashes tremendous amounts of energy. This causes a Seyfert galaxy's characteristic emission, which is extremely bright in the X-ray spectrum and emanates from a tiny central region or nucleus.

In some Seyfert galaxies, though, the emissions from this central black hole are somewhat obscured. Seyfert galaxies where the black hole's emissions come through clearly are labeled Seyfert 1 galaxies; others with less clear emissions are known as Seyfert 2 galaxies.

Previously, theorists had attributed the difference to Earth's angle of view on the galaxies. Astronomers believe a roughly doughnut-shaped ring of gas and dust, known as the torus, exists just outside the core of most galaxies. Depending on the orientation of a Seyfert galaxy, the central black hole might be visible only through the obscuring effects of the torus.

  HST
This false-color image at visual wavelengths observed with the Hubble Space Telescope is 14,000 light years across. Credit: M. Malkan (UCLA) and NASA
 
When Levenson and other researchers studied 14 Seyfert galaxies with new and archived data from two orbiting X-ray observatories, though, they found evidence that suggested another factor might be helping obscure the central black holes: regions of frequent, rapid star formation known as starbursts.

Previous research had identified the galaxies they studied as having some characteristics of starburst galaxies. These galaxies can be as bright as Seyferts, but most of their emissions come from a region surrounding the galactic nucleus, where new stars are being born, instead of the nucleus itself.

Through analysis of the X-ray emissions from the galaxies, Levenson's research team amassed strong evidence that the galaxies had both the region of high star formation characteristic of a starburst and the central supermassive black hole of a Seyfert.

The link suggests some tantalizing new ideas about galactic evolution to Levenson and her co-researchers.

"Something must be tugging on the materials in a Starburst region and increasing the chance that the gases there will collect and form stars," Levenson said. "In the Seyfert galaxies, the gravity of the central black hole could be helping to do this."

Starburst galaxies in the new study also were near to or interacting with other galaxies, a finding that supports a prior hypothesis that interactions with nearby galaxies could provide the necessary agitation for starbursts.

  X-ray
This X-ray image taken with the ROSAT satellite is 30,000 light years across. In this false-color representation, the bright nucleus of the galaxy is yellow. Credit: N. Levenson (Johns Hopkins University) and NASA
 
The new results could further flesh out the connection between Seyfert 1 galaxies and Seyfert 2 galaxies.

"Another factor obscuring the central black holes' emissions in Seyfert 2 galaxies may be the starburst region," Levenson explained. If so, then the latter stages of the starburst, with multiple stars dying explosive deaths that sweep away gas and dust, may help produce the clear view of the galaxy's core seen in Seyfert 1 galaxies.

"We are beginning to realize that starbursts and Seyferts may exert a powerful influence on each other," says Kimberly Weaver, an astrophysicist at NASA's Goddard Space Flight Center and a co-author of the research. Starbursts may comprise a significant fraction in the energy output of active galaxies like Seyferts, and could also be boosting the output of other distant high-energy sources such as quasars.

Finally, the link between starbursts and Seyferts suggests the possibility that matter swept about by star deaths in the starburst region may help feed a Seyfert's central black hole. However, there's no direct evidence for this idea and little hope for a good test, Levenson notes.

"The disks of material surrounding a supermassive black hole and being pulled into it are only about a third of a light year in radius," Levenson said, "and starbursts are a few hundred light years out from the core." Detecting a connection between those two areas at the distance of the Seyfert galaxies, 50 million to 500 million light years away from Earth, and through the overwhelming glare of the central black hole, seems unlikely, she said.

Data for the study were obtained from ROSAT, the German/UK/US Roentgen Satellite; and ASCA, the Japan/US Advanced Satellite for Cosmology and Astrophysics. Levenson's co-authors were Timothy Heckman, a professor of physics and astronomy at Hopkins, and Kimberly Weaver, an astrophysicist at NASA's Goddard Space Flight Center.

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