E MANN

Cosmic giants and dwarfs do battle in space

Written by Teresa Belcher

Optical collision between galaxies NGC 2207 and IC 2163. Image: Stuart Rankin

Galaxy collison favours bigger guy
Larger galaxy forms stars at expense of smaller galaxy
Milkyway and Andromeda to collide in four billion years

WHO IS the winner when two galaxies hurtling towards each other on a collision course through space smash together?
The collision of cosmic proportions was considered by researchers at the International Centre for Radio Astronomy Research (ICRAR) who determined that the bigger guy always comes out on top.
Previously, astronomers thought that when two galaxies smash into each other their gas clouds get churned up and seed the birth of stars much faster than if they remained separate.
However, research headed by astrophysicist Dr Luke Davies, from the UWA node of ICRAR, suggests the outcome depends on the size of each galaxy.
“When two ‘giants’ collide, they both increase their stellar birth rate, but when one galaxy significantly outweighs the other, the ‘giant’ begins rapidly forming new stars, whereas the ‘dwarf’ suddenly struggles to make any at all,” he says.
Dr Davies says the different outcome is likely due to how long the collisions take to happen.
He says when two ‘giants’ get close they smash into each other very quickly and form a single galaxy before they use up all of their gas.

*An animation of galaxies colliding. Credit: ICRAR*

But when a ‘giant’ and ‘dwarf’ meet they take a very long time to actually reach each other, and the longer collision timescale means they have time to use up all of their star-forming gas and stop making new stars.
“I like to think of this as the dwarf staying out of arms reach of the giant for a long time, but running around so much that they eventually run out of steam—while the two giants just smash straight into each other in a flurry,” he says.
Dr Davies has studied more than 20,000 merging galaxies as part of the Galaxy and Mass Assembly (GAMA) survey.
“GAMA accurately measures the positions in space of hundreds of thousands of galaxies, so we can work out which ones are very close and more likely to collide in the near future,” Dr Davies says.
“The light emitted from these galaxies is measured at lots of different wavelengths, and from that we can work out their characteristics, such as how many stars they have and how fast they are forming new stars.”
Dr Davies says the Milky Way and our nearest neighbour, Andromeda, are like ‘cosmic tanks’ on a collision course, and in about four billion years they will merge to become a new galaxy Milkdromeda.
Notes
ICRAR is a joint venture between Curtin University and the University of Western Australia with support and funding from the State Government of Western Australia.

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