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Monday, 4 June, 2001, 17:07 GMT 18:07 UK
Spiral galaxy stays young at heart
M33 galaxy
The offending bulge in M33
By BBC News Online science editor Dr David Whitehouse

Observations of a neighbouring galaxy have thrown doubt on accepted theories about galactic structure and evolution.

Astonomers observing the spiral galaxy have realised it has a much smaller central bulge of older stars than was expected.

This finding makes us question the role of a bulge in spiral galaxy formation

Andrew Stephens, Ohio State University
Before now it was thought that the central bulge and disk of spiral galaxies were made up of stars of different ages. Astronomers thought that older stars inhabited the central bulge and younger stars were to be found in the disk.

But some of the first images of galaxy M33 have shown that it has far fewer than expected older stars in its bulging heart.

Cosmic corpulence

Astronomers have long thought that a typical galactic disk is made up of stars of all ages. The central 'bulge' of a disk normally contains old stars from the time the galaxy formed. This is one reason why studying bulges can tell astronomers about how galaxies formed and evolved.

According to current theory, spiral galaxies begin as a giant rotating mass of gas and dust, which starts out in a roughly spherical shape before the edges flatten into a disk.

As the disk forms, the gas and dust contracts, forming stars in the distinctive spiral pattern. The original spherical shape remains in the form of a "halo" surrounding the galaxy and, to a lesser extent, in the central bulge.

But astronomers have been puzzled by some of the first images from the 8-metre (26 foot) Gemini North telescope on Mauna Kea in Hawaii, which looked at the innermost 2,500 trillion km (1,550 trillion miles) of galaxy M33.

Instead of the old stars that normally populate the galactic bulge, the astronomers saw youthful and middle-aged stars and no central bulge at all.

Composite images

"This finding makes us question the role of a bulge in spiral galaxy formation," said Andrew Stephens of Ohio State University's Department of Astronomy. "If M33 doesn't have a bulge at all, then how exactly did it form? If it has young stars in its bulge, what triggered their formation?"

To scrutinise the M33 bulge the Ohio State astronomers recorded images in three infrared wavelengths and combined them to form a composite image of the galaxy.

The high-resolution images they obtained revealed bright, red, middle-aged stars, and even bright, blue, young stars, both of which should not be present in the bulge.

This is puzzling to astronomers and, if the same effect is found to be common among other spiral galaxies, it could force a reassessment of ideas about how galaxies formed.

The research was presented at a meeting of the American Astronomical Society in Pasadena, US.

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