THE INTERACTING GALAXIES

Many galaxies show evident signs of interactions with nearby galaxies. Actually, the interactions among galaxies are quite frequent, and are more likely than those among stars. In a galaxy like ours, for example, the mean distance between two stars is several million times bigger than their diameter, and therefore a collision is very unlikely, while, in a cluster, the mean distance between two galaxies can be only ten times bigger than their diameter.
In some cases, galaxies that appear to be interacting are not physically associated, but look very near in perspective.

The interacting galaxies M51 and NGC 5195. (SEDS)

 

The interaction among galaxies occurs when they are near enough come under the effect of their gravitational fields. At this point they start orbiting one around the other. the tidal force between the two galaxies tends to deform them and disgregate them, much more so if they are very near and have a small relative speed.
Only the outer regions of the galaxies are initially interested by the tidal effect; for example, if they are spiral galaxies, their arms tend to "unwind", and they partially disgregate. A well-known example is the couple NGC 4038-4039, usually called "the antennae".

The two galaxies NGC 4038-4039, the "antennae". The antenna structure is due to the "unwinding" of the spiral arms.

If the interaction goes on, the gas present in the galaxies and the stars concentrate in the nucleus. Apparently, the gas and the stars form a bar of stars and gas that crosses the galaxy in the centre, and this would explain the presence of the barred spiral galaxies. In other cases, the two galaxies can merge, forming a brighter and bigger galaxy. The modern calculation techniques allowed to simulate the encounter between two galaxies, and the evolution of their shape as the interaction proceeded. The results of the simulations seemed in accordance with the observations: in fact, there are many galaxies with a distorted morphology that resembles that of the simulations. In particular, it seems that an elliptic galaxy can be the result of the union of two disc galaxies, although there is no definite evidence. The interaction process, the deformation of the galaxies and the concentration of gas in the centre require a few hundred millions of years.

If the interacting galaxies are spiral or irregular, in other words if they contain gas, the increase of the star formation is an important effect of the interaction. As already said, in fact, during the interaction most of the gas is concentrated in the nucleus of the galaxies, at a high speed. Such gas is compressed, originating thick clouds, which can collapse, forming a large number of stars.

Apparently, if this stellar formation is very intense and concentrated in a small region of the galaxy, the evolution of the stars can lead to the formation of a huge black hole in the centre. The surrounding gas, falling on top of it, could produce enormous amounts of radiation. According to the most reliable models, this mechanism would be responsible for energy production in the active galaxies and in the quasars. The interaction events are rather frequent in a cluster of galaxies, where they are quite near. Often, there are elliptical giant galaxies in the centre of these clusters, with a much larger mass than the other galaxies of the cluster. It looks like they have grown through repeated episodes of cannibalism towards nearby galaxies, caught by the gravitational field of the central galaxy.

Hubble Space Telescope revealed huge comet-shaped gas clouds in this "cart wheel-shaped "galaxy, moving through its nucleus at very high speed. They appear like white streaks inside a blue ring; their shape is probably due to the collision between faster gas and slower gas. The unusual shape of this galaxy was in fact produced by the frontal collision with a smaller galaxy, which happened approximately 200 million years ago. (HST)

 
 
 
 

ANIMATIONS

Interacting galaxies, MPEG, 465 Kb.

In this picture you can see the contact between two galaxies with equal mass. The response of the gas to the perturbation is highlighted by colour: red for the stronger stellar formation, blue for the weaker.

Interacting galaxies, MPEG, 1223 Kb.

The contact between a disc galaxy with a high amount of gas and a dwarf galaxy, with no gas. The latter is not shown, but you can see the response of the gas of the former to its gravitational field (the coloured dots represent the gas clouds).