Scientists Can Finally Determine The Accurate Shape Of Any Galaxy

For over 90 years, astronomers have tried to determine the three-dimensional shape of a galaxy accurately, just by looking at it. It sounds like a simple premise — look at an object and determine its shape, right? Apparently not.

Determining the shape and dimensions of a galaxy is a task no astronomer has been successful at, and there is a simple reason for this. Galaxies are a constantly moving, complex system of planetary bodies. They are constantly rotating around their own axis and also hurtling through space in an effort to get away from each other because of the endlessly expanding universe.

Since the dawn of time, man has looked up at the stars and wondered about them, and since we found out that galaxies existed, we have been trying to figure out their shape.

Dr Caroline Foster and her team from the University of Sydney might have solved this seemingly trivial issue with an elegant solution. They have linked the shape of a galaxy to its spin and rotation speed.

"This is the first time we've been able to reliably measure how a galaxy's shape depends on any of its other properties — in this case, its rotation speed," said Dr Caroline, who led the team.

The study was published in the journal Monthly Notices of the Royal Astronomical Society .

Galaxies come in a plethora of shapes. They can be a spiral, a helix, a ball of stars or football shaped. Studying such a diverse, random assortment of bodies in our cosmos can be cumbersome.

"I guess it's the inaccessibility that drew me to this study," Dr Foster said in an article by the Sydney morning Herald.

The complexity of this arises from the fact that you cannot look at the entire galaxy, or even a cross section of it. All you see is it’s alignment with respect to its motion through space and around its own axis.

This problem was overcome by studying individual stars within a galaxy.

The images of the galaxies the researchers already had was combined with data from the Coonabarabran-based multi-object integral field spectrograph known simply as SAMI, a device that can be used to measure the movement of stars within a galaxy.

Tracking stars within a galaxy, which wasn’t possible until the spectrograph was invented, gave the researchers the answers they needed.

The different directions in which stars move within a galaxy, when observed over time can help you find a pattern and where in the galaxy the star is placed. When you study enough stars from the same galaxy, you start uncovering the actual shape of the galaxy.

Simple physics dictates that in a galaxy that spins, one side moves towards the observer and one moves away. Finding out the direction of the stars movement will tell you if the star is travelling towards or away from you, and help establish the direction of rotation of the galaxy.

The speed at which each galaxy spins is determined by correlating the speeds of the individual stars and the direction of their motion.

"And among spiral galaxies, which have disks of stars, the faster-spinning ones have more circular disks," said team member Professor Scott Croom of the University of Sydney in a report by phys.org.

They also found out those galaxies that rotate very slowly or do not rotate at all, formed a wider range of shapes. They spread out more and formed shapes like oblate spheroids and triaxial ellipsoids, which are shapes we do not come across often.