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Svelte Spiral Sparks Questions about Galaxy Formation

galaxy
Image: Courtesy of OHIO STATE UNIVERSITY

Images from the Gemini North telescope on Mauna Kea in Hawaii show that the nearby spiral galaxy M33 is slimmer than astronomers had suspected. The new findings, presented yesterday at a meeting of the American Astronomical Society in Pasadena, Calif., may force researchers to revise current theories about the formation of such galaxies.

In most spiral galaxies, curved arms of gas, dust and stars radiate out from a spherical nucleus of stars. The galaxy thus resembles a flattened disk with a bulging center. Current models of spiral galaxy formation posit that the galaxies start out as a ball of gas and dust. Later, the edges flatten out and form the spiral arms, but the center retains a bulge, which normally contains the old stars that date back to when the galaxy formed. Previous investigations had purportedly located the bulge in M33, but when Andrew Stephens and Jay Frogel of Ohio State University examined the new images, they found a svelte center. It's as if the outer disk extended to M33's core, Stephens says. Furthermore, the infrared imaging of the galactic center revealed bright red middle-age stars and bright blue young stars (see image), instead of the old stars that usually occupy the bellies of spiral galaxies.

As is often the case, the new results raise more questions than they answer. "If M33 doesn't have a bulge at all, then how exactly did it form," Stephens muses. "If it has young stars in its bulge, what triggered their formation?"

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