ASTRONOMERS FIND MULTIPLE GENERATIONS OF STAR FORMATION IN CENTRAL STARBURST RING OF A BARRED SPIRAL GALAXY

TUSCALOOSA, Ala. - Astronomers are announcing today that they have found evidence for multiple generations of star formation in a ring near the center of a barred spiral galaxy. The report is being presented today by Drs. Ronald Buta, Deborah A. Crocker, and Gene G., Byrd, and graduate student Patrick Treuthardt, to the American Astronomical Society meeting in Atlanta, GA. The result is of special interest because it may shed light on how star formation proceeds in the unusual conditions near the center of a barred spiral galaxy.

The star-forming ring is located in a galaxy known as NGC 1326, lying 62 million light years from Earth in the constellation Fornax. NGC 1326 is one of many known barred disk-shaped galaxies showing rings as part of their structure. NGC 1326 in fact has four such rings, but most of the star-forming action seems to be taking place in its smallest ring, which hovers only 1,500 light years from the nucleus. The galaxy itself is more than 90,000 light years in diameter. Buta and his co-workers used the Hubble Space Telescope to observe the central ring in NGC 1326 at high resolution to examine how the new stars are distributed in the ring, and to try and measure the ages of the stars in order to gauge how the ring has evolved.

"With the excellent resolution of the Hubble Space Telescope, we were able to see details in the ring that we could not hope to see in a normal ground-based picture," said Buta. "Our images revealed nearly a thousand discrete point sources in the ring, many of which are unresolved clusters of massive young stars. From the colors of these point sources, we have deduced that most are less than 10 million years old, some are between 20 and 90 million years old, and a few are as much as 200 million years old. The existence of these older clusters suggests that a star formation episode occurred in the ring 200 million years ago, and it may very well have been the first episode.''

Galaxies come in a variety of shapes, from elliptical to spiral to irregular, but no shape has intrigued Buta more than the ring patterns seen in some galaxies. Like the rings of Saturn, ring patterns in galaxies are interesting because a ring is such a well-organized shape and because rings provide clues to how galaxies evolve.

"There is considerable evidence that the ring-like patterns we see in many galaxies are connected to a phenomenon known as orbital resonance," said Buta. "Many highly-flattened disk-shaped galaxies, over the course of their lives, develop a feature known as a bar - an elongated mass of stars moving together as a pattern that can persist for a very long time. As the pattern rotates, the orbits of stars and gas clouds in the galaxy's disk are perturbed by the bar's gravitational pull, and may become distorted from near-circular shapes. In some regions of a galaxy's disk, stars and gas clouds will feel the grip of the bar more strongly than in other regions because they are moving in step with the bar pattern. These are the special regions of orbital resonance."

Buta said that near an orbital resonance, the bar can cause gas and dust clouds to collect into a ring-shaped pattern. This collecting tendency causes pile-ups of clouds, leading to the formation of rings of new stars. "Most rings that we see in galaxies are sites of active star formation, and this is a major observation supporting the resonance theory of ring formation," Buta explained. The pile-up of gas clouds in a small ring around the center of a barred galaxy can be so significant that periodic starbursts may occur. The existence of multiple generations of star formation in the central ring of NGC 1326 may be supporting this idea.

Buta notes that the result is a little controversial because the ring clusters are very faint and are being viewed against the extremely bright background of the galaxy's central region. It is difficult to distinguish distinct generations of stars from simply continuous star formation over a period of 200 million years. Also, colors can be ambiguous for estimating ages. Still, the results provide some of the most promising evidence yet obtained that starburst rings in barred galaxies have a complex star formation history.

The team's images also revealed a significant amount of dust inside and outside of the ring, concentrated in irregular lanes that extinguish and redden some of the background starlight. Dust is composed of fine particles made of elements such as carbon and silicon, and is often found in star-forming regions. Buta notes that the significant amount of dust inside one half of the ring suggests that that side of the galaxy is tipped towards us.

Buta further notes that many people have seen an orbital resonance without actually knowing it. "Whenever you view the rings of Saturn through a small telescope, you can usually see a dark gap in the rings where there appears to be little ring material," said Buta. "This gap is known as the Cassini Division and it represents a region in the rings where material moves in orbital resonance with Saturn's innermost moon Mimas. Rather than producing a pile-up of material, the gravitational pull of Mimas cleared a gap because of the solid nature of the ring material. In a galaxy, the tenuous clouds of gas and dust do not bounce off each other in a resonance region like solid particles, but coalesce and form a bright ring of new stars instead of a gap. The result is different, but the physical process is the same," he explained.

The UA team's work has been supported by grants from the National Aeronautics and Space Administration and by the National Science Foundation. A color photograph of NGC 1326's star-forming ring based on the Hubble Space Telescope observations, as well as a ground-based photograph of the whole galaxy, can be found at the following Web site: http://www.astr.ua.edu.

Contact: Lance M. Skelly
Office of Media Relations, 205/348-3782
E-mail: lskelly@ur.ua.edu

Source: Dr. Ronald Buta, Professor of Astronomy, 205/348-3792