Galaxy Spin Rate Determines if It’s Clumpy Galaxy or Spiral

December 16, 2015

Some galaxies appear clumpy without a pattern or distinguishable features. It was theorised this was caused by the high levels of gas these galaxies have, but a study led by Danail Obreschkow, from the University of Western Australia, Crawley, Australia, suggests it’s actually the low stellar angular momentum of clumpy galaxies that determines their appearance.

Spiral galaxies' (left) and clumpy galaxies' (right) shapes compared. Clumpy galaxies have more star-forming gas (red). Credit: Danail Obreschkow, ICRAR/Hubble Space Telescope

By observing four galaxies, Obreschkow and his team concluded that galaxies like the Milky Way have their spiral appearance because they rotate three times faster than clumpy galaxies. The high amount of gas, previously theorised to be the cause, has only a complementary role, according to this study. The fact that high levels of gas are clumped together and not spread out in a disk like in spiral galaxies, is cause for rapid star formation in clumpy galaxies compared to spiral galaxies of the same mass.

"While the Milky Way appears to have a lot of spin, the galaxies we studied here have a low spin, about three times lower," said Obreschkow. "The clumpy galaxies produce stars at phenomenal rates. A new star pops up about once a week, whereas spiral galaxies like our Milky Way only form about one new star a year."

Astronomers sampled four out of 95 'dynamo' galaxies using data from Keck and Gemini observatories. These galaxies turned out to have marginal stability and high rate of star production. Clumpiness and high star birth-rate are something that resembles early galaxies, when our Universe was still young.

The further back in time we want to look, the more distant are the galaxies we want to observe. This makes measurements of angular momentum of these early galaxies impossible at the time.

However, Obreschkow and his team theorise, based on observation of these four local, slow-rotating galaxies, that earliest clumpy galaxies rotate two times slower than older, spiral galaxies. Early clumpy galaxies increase their angular momentum because of cosmic expansion and become spiral galaxies with time.

Source 1 (PDF file), Source 2

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