How Can Galaxies Lose That Bulge? By Spinning Faster

By Shweta Iyer on February 28, 2014 5:18 PM EST

galaxy
This is a photo of Galaxy M101, an example of a 'flat' galaxy. (Photo: Fabian Neyer, Stemwarte Antare)

Running fast to stay flat and toned is not just a mantra for humans. Believe it or not it applies to the cosmos as well. Australian astronomers led by the International Center for Radio Astronomy Research (ICRAR) in Perth have discovered that the rate at which they spin determines why some spiral galaxies are fat and bulging while others are flat discs, according to a press release Friday.

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The study published in the Astrophysical Journal found that fast rotating spiral galaxies are flat and thin while galaxies of the same size that rotate slower are fatter. ICRAR Research Associate Professor Danail Obreschkow, from The University of Western Australia, said that the differing size and look of galaxies has been a much debated topic."Some galaxies are very flat discs of stars and others are more bulging or even spherical. Much of the last century of research has been dedicated to understanding this diversity of galaxies in the Universe and with this paper we've made a step towards understanding how this came about by showing that the rotation of spiral galaxies is a key driver for their shape," said Dr. Obreschkow.

As part of the study the astronomers used data from a survey called THINGS and studied 16 galaxies, which were between 10 million and 50 million light years from Earth. The data was collected at the Karl G. Jansky Very Large Array in the United States, one of the most famous radio telescopes in the world and a significant pathfinder for the Square Kilometre Array. "The THINGS survey shows you the cold gas in the galaxies, not only where it is but how it moves," Dr.Obreschkow said."That's a crucial point if you want to measure the spin, you can't just take a photograph, you have to take a special picture that shows you the motion."

According to the research, the shape of a spiral galaxy is determined by both its spin and its mass, and both quantities will remain the same if the galaxy is undisturbed for billions of years.Dr. Obreschkow said that the formation of galaxies looks a bit similar to a carousel made of an elastic disc."If the carousel is at rest, the elastic disc is quite small," Dr. Obreschkow said. "But when the whole thing is spinning the elastic disc becomes larger because it's feeling the effects of centrifugal force."

The Milky Way's shape, which can be seen in the night sky, also appears as a relatively flat disc with only a small bulge. Dr. Obreschkow said, "The white band of the Milky Way across the sky is a relatively thin band of constant thickness. However when you look right at the center near the Sagittarius constellation you can actually see a thickening of the Milky Way, which is the bulge."

Dr. Obreschkow and Swinburne University Professor Karl Glazebrook, who are authors of the study, were for the first time able to measure the effect of spin on galaxies more than ten times better than before.

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