The galaxy NGC 1291 is about 12 billion years old—and that’s old. So what’s it doing with a ring of newborn stars around it? In this newly released image from NASA's Spitzer Space Telescope, trapped gas at the galaxy’s outskirts have triggered star birth.
Located 33 million light-years away in the Eridanus constellation, NGC 1291 is known as a barred galaxy because its central region is dominated by a long bar of stars—which appears as an "S” in the blue circle in the image above. (Our galaxy has a bar too, though not as prominent.) The stellar bar formed early in the galaxy’s history, and as it stirred material around, stars and gas were forced into large, non-circular orbits. This created areas where gas was compressed—called resonances—which trigger the formation of new stars.
"The rest of the galaxy is done maturing," Kartik Sheth of the National Radio Astronomy Observatory explains in a news release. "But the outer ring is just now starting to light up with stars."
In young, gas-rich galaxies, stellar bars drive gas toward the center, feeding star formation. But as the galaxies age, and the star-forming fuel runs out, the central regions become inactive, and star-formation shifts to the galaxy’s outskirts. There, spiral density waves and resonances produced by the central bar convert gas into stars.
In the image above, shorter-wavelength infrared light appears blue, longer-wavelength light is red. The stars in the galaxy’s center (blue) are older, since most of the central region’s gas was already used up by earlier generations of stars. The outer ring (red) is the resonance area where trapped gas has ignited a star-forming frenzy. The new stars are heating up dust that glows with infrared light.
To better understand how stellar bars influence galaxies, Sheth and colleagues are analyzing the structures of more than 3,000 galaxies in our local neighborhood as part of the Spitzer Survey of Stellar Structure in Galaxies, or S4G. "The bars are a natural product of cosmic evolution, and they are part of the galaxies' endoskeleton,” he says. “Examining this endoskeleton for the fossilized clues to their past gives us a unique view of their evolution."