When cosmic giants meet galactic dwarfs

When two different sized galaxies collide, the larger stops the smaller one making new stars, says a study of more than 20,000 merging galaxies.

When two galaxies of the same size collide, both galaxies produce stars at a much faster rate.

"Our nearest major galactic neighbour, Andromeda, is hurtling on a collision course with the Milky Way at about 400,000 kms per hour," said Luke Davies, astrophysicist from the University of Western Australia.

But there is no reason to panic yet since the two would not smash into each other for another four billion years or so.

The study appeared in Monthly Notices.

Earlier, astronomers thought when two galaxies smash into each other, new stars are born much faster than if they remained separate.

But the study shows whether a galaxy forms stars more rapidly in a collision, or forms any new stars at all, depends on if it is the big guy or the little guy in this galactic car crash.

"When two galaxies of similar mass collide, they both increase their stellar birth rate," Davies said.

"However when one galaxy significantly outweighs the other, star formattion rates are affected for both, just in different ways. The more massive galaxy begins rapidly forming new stars, whereas the smaller galaxy suddenly struggles to make any at all."

"This might be because the bigger galaxy strips away its smaller companion's gas, leaving it without star-forming fuel or because it stops the smaller galaxy obtaining the new gas required to form more stars," Davies said.

"So when the Milky Way collides with Andromeda, they will begin to affect each other's star formation, and will continue to do so until they eventually merge to become a new galaxy, which some call 'Milkdromeda'."