Black Holes Linked to Star Births

A new study has found that what were once thought of as two distinct types of galaxies may in fact be one and the same. Nancy Levenson and her colleagues at Johns Hopkins University reported on Monday at a meeting of the American Astronomical Society that high rates of star formation, the hallmark trait of so-called starburst galaxies, are also extremely common in Seyfert galaxies, characterized by supermassive black holes at their centers. NGC 5135, pictured at the right, is an example. In addition, they described a mechanism for how the two phenomena could be linked by more than coincidence. "The process that makes the central black hole in a galaxy grow more massive may help trigger these bursts of significant star formation," Levenson notes. "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 are typically so bright because their black holes unleash huge amounts of x-ray energy as they suck up surrounding stars and gas. Some, however, labeled Seyfert 2 galaxies, do not seem to emit such great amounts of x-rays. Astronomers had assumed that the emissions were obscured by the angle of view afforded from Earth. But when Levenson's team analyzed 14 Seyfert galaxies using new and archived data from two orbiting x-ray observatories, they reached a different conclusion: starburst areas may partially block out the black holes in Seyfert 2 galaxies. "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 adds. "In the Seyfert galaxies, the gravity of the central black hole could be helping to do this." They further speculate on the relationship between Seyfert 1 and 2 galaxies: once the starburst period ends in the latter kind, the explosive deaths of multiple stars may sweep away gas and dust to reveal clearly the core in the first variety.