Snickers provides clue to galactic formation

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Science Editor

Art may imitate life, but now cosmology is imitating confectionery. Inspired by Galaxy and Mars bars and Milky Ways, Dr Michael Merrifield of South-ampton University has come up with the "Snickers" peanut-bar hypothesis, giving for the first time a three-dimensional picture of the central parts of spiral galaxies.

Together with Dr Konrad Kuijken, of Groningen University in the Netherlands, Dr Merrifield has been studying the structure of spiral galaxies, such as our own Milky Way. Their work, which will be reported tomorrow at the United Kingdom National Astronomy Meeting in Liverpool, has revealed that where there is a bar, there is also a central bulge shaped like a peanut.

Although most stars congregate on one plane, forming a thin disc like a gramophone record, the centre thickens up into a "bulge" of stars and the new hypothesis provides an explanation for how these central bulges may have formed.

Around a third of the bulges have a double-lobed appearance, rather like a peanut in its shell. This is only visible in galaxies which can be seen edge-on. About a third of spiral galaxies (one of the most common formations) which can be seen face-on show a central bar - like distortion in the starlight near their centres.

The fact that peanut-shaped bulges and bars occur in similar parts of galaxies has led astronomers to advance the "Snickers hypothesis" - that the peanut structures are actually formed in the galactic bars.

Since no galaxy can be viewed both face-on and edge-on, we cannot witness both phenomena in a single structure. However, Drs Merrifield and Kuijken studied 10 edge-on galaxies, some of which had round bulges and some peanut- shaped bulges, and by looking at the orbital motion of the gas and stars they found unequivocal evidence that the peanut ones also harbour bars.

A bar-shaped mass of stars will alter the gravitational pull of the galaxy as a whole, distorting the motions of the gas and stars so that they follow complicated elliptical orbits. Using the William Herschel Telescope in La Palma, Canary Islands, to map the spectra of light emitted, Dr Kuijken and Dr Merrifield found that galaxies with round bulges had gas and dust on circular orbits - implying that they did not contain bars. On the other hand, material in galaxies with peanut-shaped bulges followed the elliptical orbits characteristic of a bar.

Computer simulations of galaxies have shown that thin discs of stars are not stable entities, and that they quite rapidly produce bars at their centres. These bars are, in turn, unstable and start to buckle, bending out of the plane of their host galaxy.

This bending back-and-forth fattens the bar perpendicular to the disc of stars, forming a peanut-shaped structure. It is apparently these fattened bars that Drs Kuijken and Merrifield have detected in their study. Ultimately, the bar will dissolve entirely, leaving just the fattened structure at the centre of the galaxy. Perhaps, they therefore suggest, all galactic peanuts originally formed from buckling bars.