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Fast-spinning galaxies beat the bulge

Paul Sutherland, Feature writer
Mar 4, 2014, 8:00 UTC

Sen—The mystery of why some spiral galaxies are much fatter than others has been solved — and as with humans it seems to depend on how much exercise they get as they rotate.

Research shows that those that spin rapidly tend to be flat or “slim”, while those rotating slowly are fatter, with a bigger bulge towards the middle.

Simple models of spiral galaxies, that classic pattern of a city of billions of stars, often compare them to two fried eggs back to back, with their yolks representing the central bulge.

In reality, some spirals are very flat, such as Messier 101 (M101) in the constellation of Ursa Major. Others, like M81, shown in our main picture, are markedly fatter.

The research was led by the International Centre for Radio Astronomy Research (ICRAR) in Perth, Western Australia, and has been published in the latest issue of the Astrophysical Journal.

ICRAR Research Associate Professor Danail Obreschkow, from The University of Western Australia, said it had long been a matter of debate why galaxies look so different to each other. He said: “Some galaxies are very flat discs of stars and others are more bulging or even spherical.


M101 rotates rapidly and so is a much flatter spiral galaxy. Credit: Hubble/NASA/ESA

“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.”

In their study, the researchers looked at 16 galaxies lying between 10 million and 50 million light-years from Earth, using data from a survey carried out by the Karl G. Jansky Very Large Array of radio telescopes in New Mexico, called The HI Nearby Galaxy Survey (THINGS).

Dr Obreschkow said: “The THINGS survey shows you the cold gas in the galaxies, not only where it is but how it moves. 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.”

Dr Obreschkow said the shape of a spiral galaxy is determined by both its spin and its mass and if you leave a galaxy on its own for billions of years both quantities will stay the same.

He compared the way galaxies are formed to a carousel made of an elastic disc, saying: “If the carousel is at rest, the elastic disc is quite small. But when the whole thing is spinning, the elastic disc becomes larger because it’s feeling the effects of centrifugal force.”

Our own Milky Way is a relatively flat disc with only a small bulge, which becomes apparent when you view the Milky Way towards the centre. 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 his co-author, Professor Karl Glazebrook, of Swinburne University, Australia, were able to measure the effect of spin on galaxies more than ten times better than anyone previously.