Contact: Barbara Ryden, (614) 292-4562
Written by Pam Frost, (614) 292-9475
[email protected]
STUDY SHOWS HOW BLACK HOLES CHANGE SHAPE OF GALAXIES
COLUMBUS, Ohio -- A study by an Ohio State University astronomer and her colleagues is bolstering the theory that elliptical galaxies change shape as they age -- likely the handiwork of black holes.
Barbara Ryden, associate professor of astronomy, suspected that elliptical galaxies begin their lives shaped like squashed ovals and gradually widen at the center, becoming nearly spherical after billions of years.
Astronomers believe black holes reside at the center of these galaxies, she explained, and a black hole could sculpt an elliptical galaxy into a round shape over time by scattering stars from the center of the galaxy evenly in all directions.
When Ryden compared the shapes of elliptical galaxies to their ages as calculated by two colleagues at the University of Birmingham, England, she confirmed her suspicion that older elliptical galaxies are rounder. She also saw evidence that widening begins inside, near the center of the galaxies -- exactly what she expected to see if a black hole was responsible.
"Over time, elliptical galaxies become spherical from the inside out," Ryden said.
Ryden presented this material in a poster session on June 6 at the meeting of the American Astronomical Society in Rochester, NY.
Her University of Birmingham colleagues, Duncan Forbes and Alejandro Terlevich, determined the ages of 50 elliptical galaxies. The two have since submitted a paper on their findings to the Monthly Notices of the Royal Astronomical Society.
Ryden charted the shapes of these 50 galaxies relative to the ages that Forbes and Terlevich had calculated, which averaged about 8 billion years. She found that galaxies younger than 8 billion years were flatter than older galaxies, and that older galaxies became more spherical -- starting with a bulging shape in the middle that gradually spread outward.
The elliptical galaxies that catch Ryden's eye probably don't look that interesting to most people -- fine sprays of stars shaped into roundish blobs that somewhat resemble a giant cotton ball. Elliptical galaxies lack the well-defined, spiral-shaped arms of their showier galactic brethren.
"Elliptical galaxies seldom make their way into pretty picture books of galaxies, because they're -- let's face it -- boring in appearance compared to a colorful spiral galaxy," Ryden admitted.
Still, she said, elliptical galaxies present some interesting examples of galactic cannibalism -- what happens when two galaxies flying through space venture too close, and the gravity of one engulfs the other.
A little over 3 billion years from now, our Milky Way is slated for that fate, Ryden said. One of our nearest neighbors, the Andromeda galaxy, is 12 billion billion miles away but closing fast at more than 150 miles per second. Both the Milky Way and Andromeda are spiral-shaped galaxies now, but astronomers believe the resulting mish-mash of stars, dust, and gas after the collision will resemble an ellipse.
"A car crash may begin with two pretty, well-designed cars but it ends with a heap of tangled rubble," Ryden said. "Elliptical galaxies are sort of the galactic equivalent of a heap of tangled rubble."
Most newly-formed elliptical galaxies measure about twice as long as they do wide, she explained. If stars follow elongated orbits where they travel away from the central black hole and back again, it makes sense that the gravity from the black hole will eventually pull them from their orbit and kick them off in a random direction.
The end result, Ryden said, is that material in an elliptical galaxy becomes "scrambled," or randomized. Over time, the flatness disappears and the galaxy becomes rounder.
Ryden's study may help explain another mystery -- why some elliptical galaxies have bright centers packed with stars and others don't.
Her research shows that younger, flatter elliptical galaxies tend to have bright centers. Older, more spherical elliptical galaxies have central cores which aren't as tightly packed with stars. She speculated that as a black hole scatters stars from the center of an elliptical galaxy, it might be able to empty the center of stars over time.
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