Spiral Galaxy M81 is an example of a fat bulgy galaxy
February 28, 2014

Spin Determines Shape Of Spiral Galaxies

Brett Smith for redOrbit.com - Your Universe Online

A pair of Australian astrophysicists has shown that the shape of a galaxy is determined by how fast it spins, according to a new report in The Astrophysical Journal.

Part of “The Evolving Universe” research series at the ARC Centre of Excellence for All-sky Astrophysics (CAASTRO) in Australia, the new study concluded that fatter spiral galaxies are relatively slow spinning, while thinner spiral galaxies are rotating are higher speeds.

Study author Danail Obreschkow, from the University of Western Australia, noted that the reason behind the shape of spiral galaxies has been heavily debated.

“Some galaxies are very flat discs of stars and others are more bulging or even spherical,” he said. “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.”

Obreschkow and his colleague Karl Glazebrook, a professor at Swinburne University, looked at 16 galaxies — between 10 million and 50 million light years from Earth. The duo used data from The HI Nearby Galactic Survey (THINGS), which was conducted at the National Radio Astronomy Observatory (NRAO) facility based in New Mexico.

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

He added that the shape of a spiral galaxy is dependent upon its rotation and size, both of which stay exactly the same for billions of years. Obreschkow said spiral galaxies are essentially elastic carousels.

“If the carousel is at rest, the elastic disc is quite small,” the astrophysicist said. “But when the whole thing is spinning the elastic disc becomes larger because it’s feeling the effects of centrifugal force.”

Our own spiral galaxy is considered to be a relatively flat disc with a slight bulge – which can be seen across the night sky.

“The white band of the Milky Way across the sky is a relatively thin band of constant thickness,” Obreschkow said. “However when you look right at the centre near the Sagittarius constellation you can actually see a thickening of the Milky Way, which is the bulge.”

In September, researchers at Iowa State University and IBM revealed that spiral galaxies and other types of disk galaxies grow out of an irregular, clumped appearance.

“In galaxy disks, the scars of a rough childhood, and adolescent blemishes, all smooth away with time,” said Curtis Struck, and Iowa State professor of physics and astronomy.

The gravity of interstellar gases and new stars change the orbits of stars within the young galaxy. Over time these effects also produce a smooth fade in brightness from the center of a galaxy to its edge.

“This process takes a few hundred million years to a few billion years,” Struck said.