Why some spiral galaxies are fat and bulging while others are flat discs

Astronomers have found the reason behind some spiral galaxies being fat and bulging while others being just are flat discs, claiming that it's all about how fast they spin.

The research, led by the International Centre for Radio Astronomy Research (ICRAR) in Perth, found that fast-rotating spiral galaxies are flat and thin while equally sized galaxies that rotate slowly are fatter.

ICRAR Research Associate Professor Danail Obreschkow, from The University of Western Australia, said that with this paper they'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.

The study looked at 16 galaxies - all between 10 million and 50 million light years from Earth - using data from a survey called THINGS.

Dr Obreschkow said that the survey shows them the cold gas in the galaxies, not only where it is but how it moves.

He said that's a crucial point if they want to measure the spin, they can't just take a photograph, they 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 said the way galaxies are formed looks a bit similar to a carousel made of an elastic disc.

Dr Obreschkow said that 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.

The study has been published in the prestigious Astrophysical Journal.