Tuesday, December 20, 2005

Tidal dwarf galaxies

Not all galaxies in the universe are as large or as distinctive in appearance as the majestic spirals such as our own Milky Way or the Andromeda galaxy (M31). Many, known as dwarf galaxies, are small and shapeless, with perhaps 1/1000 the mass of the Milky Way. Some of these formed soon after the big bang and have changed little since then. Others may have formed much later as a byproduct of collisions between much larger galaxies. These are known as tidal dwarf galaxies.

Is there any way to tell which is which?

Cornell astronomers investigate cosmic forces that produce new galaxies
To understand which dwarf galaxies are tidal in origin and how those galaxies differ from primordial dwarf galaxies, Cornell researcher Sarah Higdon and her colleagues studied a system called NGC 5291, which is 200 million light years from Earth and stretches a distance roughly four times the span of the Milky Way. At the system's center are two colliding galaxies; behind them trail a string of much smaller dwarfs.

By looking for signs of strong star-forming activity, the researchers found that it was taking place mainly in the cast-off tidal dwarfs, rather than in the colliding galaxies themselves. This enabled them to recognize characteristic properties of the tidal dwarfs:
the team found that the tidal dwarfs show strong emission from organic compounds, found in crude petroleum, burnt toast and (more relevantly) stellar nurseries, known as PAHs -- for polycyclic aromatic hydrocarbons. And for the first time, the researchers detected warm molecular hydrogen -- another indicator of star formation, and one that has never before been directly measured in tidal dwarf galaxies.

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