UA, Bevill Astronomers Use Hubble Space Telescope to Reveal Spiral Galaxy with ‘Backward’ Arms

Remarkable Hubble Space Telescope images of the disk of a distant spiral galaxy will be presented by astronomers from The University of Alabama and Bevill State Community College on Jan. 8 at the American Astronomical Society meeting in Washington, D.C.

Tarsh Freeman, professor at Bevill State Community College, and Drs. Gene Byrd and Ron Buta, professors of astronomy at The University of Alabama, will present the images.

The galaxy, known as NGC 4622, lies 200 million light years away in the constellation Centaurus. The images are of special interest because they solve a mystery about the galaxy that has lingered for more than a decade. The mystery was connected to the direction the galaxy’s spiral arms wind outward.

“Fifteen years ago I noticed something unusual in a picture of NGC 4622 from a well-known undergraduate textbook,” said Byrd. “The galaxy was presented in the text as a superb example of a spiral galaxy, with two bright spiral arms that open outward in a clockwise direction. However, in the inner parts of the galaxy, I noticed another spiral arm that wound in the opposite sense to the two others. I immediately suspected that NGC 4622 has leading spiral structure, a phenomenon which, up until that time, had not been definitively recognized in any galaxy.”

It has long been believed that most spiral arms seen in galaxies are trailing, meaning they wind outward opposite the direction of rotation of the disk material, something like what one sees while stirring cream into a cup of coffee. A leading arm does the opposite, opening outward into the same direction as the rotation of the galaxy’s disk.

Byrd teamed up with colleague Buta in the early 90s to try and solve the mystery of which arm or arms lead in NGC 4622. To do this, they had to determine which way the galaxy spins: clockwise or counterclockwise on the sky. Although they obtained much follow-up data with telescopes in Chile, these ground-based data could not answer this question.

“The problem,” said Freeman, “is that NGC 4622 is like a dinner plate on the table. It is tipped very little to the line of sight. Determining which arms lead in NGC 4622 requires knowledge of which side of the galaxy is tipped toward us and which half of the galaxy recedes from us as the stars rotate around the center.

“With our ground-based data, we were able to determine which half recedes but we were unable to determine which side is tipped toward us. Hubble Space Telescope observations of this galaxy were needed to determine which side is nearer because they could show details previously hidden from us by Earth’s atmospheric blurring. In particular, they show dark silhouettes of dust clouds in the disk of the galaxy seen against the starlight of the bright spherical central bulge of stars.

“Because the disk of NGC 4622 is slightly tilted, one side is nearer to us than the other. On the near side, we view the bulge through the dust, while on the far side we view the dust through the bulge. This difference makes the dust silhouettes stand out more clearly on the near side.”

“The effect is so obvious and unambiguous in our images,” Buta said, “that we knew the answer about which arms lead as soon as we saw the HST images on our computer screen. The answer was not, however, what we expected.”

“We had thought since 1993 that the outer pair of arms trailed,” said Byrd. “These two arms were strong and there were good theoretical reasons to believe they were trailing. However, in order for them to be trailing, the galaxy would have to be spinning counterclockwise. The Hubble Space Telescope data told us that the galaxy was, in fact, spinning clockwise. This gave the surprising result that the two outer arms have the leading sense, not the weaker inner arm. We were absolutely stunned by this result.”

Why does NGC 4622 have two strong leading arms when most galaxies have trailing arms? Byrd, Buta, and Freeman think they know the answer. “We have suspected for a long time that NGC 4622 has suffered from some kind of interaction with another galaxy,” said Buta. “Its two outer arms are lopsided, meaning something has disturbed it. The new HST images suggest, in fact, that NGC 4622 has consumed a small companion galaxy.

“In the center we see new evidence for a merger between NGC 4622 and a smaller galaxy. This could be the key to understanding the unusual leading arms,” he said. This research was supported by NASA and the National Science Foundation.

More images of the galaxy