COLUMBUS, Ohio -- Astronomers at Ohio State University have uncovered evidence that the inner part of our galaxy is shaped like an oval, rather than a perfect circle, as once thought.
This information will help astronomers understand how the galaxy formed and whether dramatic events such as collisions with other galaxies molded the Milky Way into the shape we see today.
Our galaxy is a pancake-shaped disk of stars, gas, and dust surrounding a thick, spherical central bulge. Over the last few years, however, astronomers have come to suspect that the bulge might not be a perfect sphere.
While analyzing data from a recent star survey, Ohio State doctoral student Glenn Tiede found that stars in the part of the disk closest to the bulge orbit the galaxy in an oval-shaped path.
“We’ve always assumed we live in a circular spiral galaxy,” Tiede said. “And now we’re getting strong indications that that’s not the case.” Tiede presented the latest results of this research at a recent meeting of the American Astronomical Society.
“What we’re turning up is pretty strong confirmation of many things about the structure of the inner galaxy that could only be inferred indirectly before,” said Donald Terndrup, associate professor of astronomy and Tiede’s advisor.
While astronomers can readily view the disks of other galaxies, which come in a variety of shapes and sizes, the dust in our disk blocks the line of sight to the inner galaxy. For this work, Tiede and Terndrup peered at stars near the center of the galaxy through naturally-occurring gaps in the dust. Tiede looked at disk stars through two of these gaps, and measured the speed at which those stars were orbiting the galactic center.
Everything in the galaxy is rotating in the same direction, so if the motion is circular, the stars on one side should be moving toward us at the same speed the stars on the other side are moving away.
“I expected that the velocities would be equal, but with opposite directions,” Tiede said. “When I found that they were not equal, at first I suspected my techniques, but I couldn’t find any mistakes. That’s when I considered what it would mean if they really weren’t equal -- that the disk isn’t circularly symmetric.”
The shape that Tiede calculated for the inner disk lines up with previous estimates of the oval shape of the bulge. The disk stars appear to travel fast along the straightest part of their orbit, and slow down as they turn the corner.
“The stars in a galaxy with a classic spiral shape go around in a circle, and in a galaxy with a bar, they go back and forth,” Tiede said. “We’re probably in an intermediate case, somewhere between the two.”
In the future, Tiede may turn his attentions to nearby stars, to see what Earth’s section of the galactic disk looks like.
Meanwhile, he and Terndrup are about to start exploring what an oval shape implies for the formation of the galaxy. If the Milky Way formed very quickly, it may have had the last 10 billion years or so to drift through space and collide with other galaxies, or absorb smaller ones, which would distort an originally circular shape.
As telescopes grow more powerful and new observing techniques emerge, astronomers will get a better picture of what’s going on at the center of our galaxy.
“These projects have a long genesis,” said Terndrup. “You get to do them over again as the technology advances.”
Contact: Glenn Tiede, (614) 292-5403;Tiede.1@osu.edu
Donald Terndrup, (614) 292-4579;Terndrup.1@osu.edu
Written by Pam Frost, (614) 292-9475;Frost.18@osu.edu