R&D News & Events

Galactic collisions give the Milky Way its arms

Author: Alan Boyd

Ever wondered how the Milky Way got its spiral arms? New computer simulations with improved data suggest they may have formed due to multiple bust-ups with another galaxy.

Galaxies are broadly classified into four types: elliptical - from perfect circles to extreme ellipses; spiral or barred spiral (such as our own Milky Way) - containing several spiralling arms around a dense centre or bar; irregular - those galaxies not easily classified into the previous types; and dwarf galaxies - small elliptical or spiral galaxies. It is with a dwarf galaxy called Sagittarius that the Milky Way appears to have had some violent encounters.

A team of astrophysicists, headed by Chris Purcell at the University of Pittsburgh in Pennsylvania, has used improved mass estimates for Sagittarius to run detailed simulations of its galactic travels over the last few billion years. Their findings, published in Nature, suggest that the Milky Way has experienced two collisions with the dwarf galaxy. Before the first collision, 1.9 billion years ago, the Milky Way was a flat disc with a central bar of stars, around which the other stars orbited. After the collision, the simulation shows the outer stars beginning to orbit the central disc in more eccentric elliptical orbits. These orbits begin to group together to form dense clumps in a spiral pattern. Over the next billion years, Sagittarius loops over the centre of the Milky Way to collide with it again. Following this collision, the Milky Way looks incredibly similar to the present day.

The widespread occurrence of galactic collisions in the cosmos suggests this may be how spiral galaxies form. This idea contrasts with previous theories, such as that spiral-shaped ‘density waves’, rotating more slowly than the galaxy’s gas and stars, squeeze the matter into spirals.

Finally, the fight is far from over. A third collision with Sagittarius is predicted soon...in 10 million years.

Posted on Tuesday, 4th October, 2011