Imagine two identically shaped stones, thrown at equal speeds from opposite directions, and hitting each other in mid-flight. The stones collide, stop moving and fall.

Now, on a much grander scale, imagine two galaxies colliding. Astronomers expect that, like the stones, the gaseous components of the galaxies would collide, lose speed and fall toward the centers of each galaxy.

The fuzzy white areas on the outskirts of the galaxy represent the strange gas.

Now, a European research team says they have solved the apparent discrepancy. Their solution, in a nutshell, is that the galaxies' gaseous clouds were smaller and denser than what astronomers expected. This compactness lowered the probability that the particles in the clouds would collide. Without collisions, the clouds could simply zoom through each other, relatively unfettered -- like the teeth of two combs passing between each other.

Gasses, unlike solids, have a significant amount of empty space between particles. If two gases made of many particles were to collide, the probability of collisions between the particles would be high. But if the particles of the gases were to clump together -- making larger molecules -- there would be greater empty space, and thus fewer collisions if two clouds merged.

The Center of the Centaurus A Galaxy

Another research team detected the gas clouds in 1994, but could not explain how it was possible that the gas had not been pulled into Centaurus A's core.

"They noticed something must be going on which we don't understand," said Cornell University astronomer Vassilis Charmandaris, who headed up the ESO's research. "We said, 'Look, we can make gas reach those regions out there by considering that the gas is denser and not too diffuse.'"

Will the gas ever be merged into the core of the galaxy?

Probably, Charmandaris said, though not for a very long time.

Charmandaris says that his group's research could help other astronomers better understand the processes of when a large galaxy "cannibalizes" a companion galaxy.