In astronomy, what you get is what you see, or more specifically what you look for.
Galaxies that have one appearance in visible light can look entirely different in ultraviolet light, as a team of astronomers found by studying a relatively nearby galaxy called NGC 6782 with the Hubble Space Telescope.
The object is what's known as a barred spiral galaxy. It has wide-ranging spiral arms like a hurricane, and a densely populated "bar" of old stars near the center. All that is clear in visible light.
But in ultraviolet light, which has a shorter wavelength and typically signals very hot stars, a different picture emerges. A dense concentration of scorching stars, seen in blue and violet colors, is revealed as a ring around the center of the galaxy. The stars are hot because they are young, indicating that the ring represents a region of intense star formation.
The poorly understood relationship between the ring in Hubble's new UV image and the galaxy's inner bar has piqued astronomers' curiosity.
The ring is formed at the edges of a bright white bar of mostly older stars, along with some gas and dust. The bar, clear in visible light but barely discernable in the new image, touches the bluish ring just at the locations where the galaxy's two main spiral arms begin their clockwise arc outward.
What's going on?
Rogier Windhorst, an Arizona State University researcher who led the study that produced the image, said his team is not yet certain of all the interactions between the inner bar of old stars and the bright ring of young stars. But he explained for SPACE.com what he suspects is going on:
Galaxies often have a bulge of stars at their centers. Our Milky Way is a good example of this. The bulge in NGC 6782 is relatively small. Around the bulge in NGC 6782, older stars orbit in such a way that they collectively maintain the shape of a bar. The phenomenon is something like a baton in a twirler's hand -- the outer parts of the bar move faster than the parts closer to the hand.
"The bar rotates essentially like a solid body, even though it is only composed of gas and stars and dust," Windhorst said.
But some of the gas at the outer edges of the bar's reach rotates less rapidly around the galactic center.
Windhorst suggests a somewhat imperfect analogy for envisioning what happens next: Imagine a standard flat ruler to represent the bar. Place it on a soft tablecloth and rotate it. The tablecloth, representing the slower-moving outer gas, will bunch up.
"At both ends of the ruler you will see the table-cloth pile up and even form some kind of spiral structure, with the outer parts of the cloth dragging behind the rotating bar."
Astronomers call this differential rotation, and it is a recipe for some of the most violent and glorious goings-on in a galaxy.
"The gas will compress in this region which has the shape of a ring, and condensed shocked gas is the place where new young and hot stars will form," Windhorst said. These stars make up the ring seen in UV.
Just as would be seen on the cloth in your tabletop experiment, two spiral arms emanate from the points where the inner bar ends in the Hubble image.
The bar of old stars may not last forever, but instead might eventually spread out into a wider distribution.
In fact, another similar but much larger and fainter bar, which extends beyond the ring of hot young stars, may already be in the process of dissolving, Windhorst said. This larger bar, not evident in the Hubble image, ends just where the spiral arms end, and a few patches of star formation, again in blue, can be seen along this outer ring.
Windhorst stressed that the idea that the bars dissolve is only a guess. A paper on this process is being prepared by Paul Eskridge, another member of the research team.
The image was made by combining two separate exposures, taken on June 22, 2000 and June 9, 2001 and totaling 1.3 hours of observation time. The image covers an area roughly 82,000 light-years across.
The photo is part of a survey of 37 relatively nearby galaxies designed to help Windhorst and others understand the differences in nearby and distant galaxies. He said that understanding optical images of distant galaxies requires an understanding of the ultraviolet findings of closer galaxies. Why?
advertisement
