Colliding galaxies electronically

Anytime astronomers witness a collision between two large galaxies, they see only a snapshot. Galaxies are so big, and gravity is such a weak force, that collisions take hundreds of millions — if not billions — of years from first brush to final outcome. Observers can see different galaxy pairs at different stages of interaction, but this gives only a vague sense of what goes on in individual cases.

Enter the computer. Researchers program the laws of physics and then give the computer a set of initial conditions: the galaxies’ masses, velocities, and distribution of matter. The computer then chugs away to produce a simulation of the colliding galaxies.

This is exactly what Joshua Barnes of the University of Hawaii and John Hibbard of the National Radio Astronomy Observatory did for the interacting galaxy pair known as the “Mice” (NGC 4676). The rounded disks and long, tail-like streamers of stars suggested two mice playing to Russian astronomer Boris Vorontsov-Vel’yaminov, and the nickname stuck. The Mice lie about 300 million light-years from Earth in the rich Coma cluster of galaxies.

It took Barnes and Hibbard a month of trial and error — mostly error, according to Barnes — to reproduce the Mice as we see them today. They found that the two galaxies apparently sideswiped each other approximately 160 million years ago and are now heading back for a second, closer pass. The simulation predicts the two will merge into a single giant elliptical galaxy roughly 400 million years from now.

Such computer simulations do more than describe objects in the distant universe. It appears our own Milky Way Galaxy will suffer a similar fate in several billion years, when we collide with the massive Andromeda Galaxy. The two galaxies are currently approaching each other at about 60 miles per second (100 km/s). A collision seems inevitable, but lies far enough away that we don’t have to worry for a while.

This computer simulation below shows the once and future history of the colliding galaxies known as the Mice, from the start of their interaction more than 150 million years ago to their ultimate merger some 400 million years from now.