Colliding Galaxies

That was a cool spring morning. After a long stroll I was sitting in my favourite corner chair in a wayside dhaba on the Inter-Galaxy Highway No. 1951. The morning edition of Times of Space lay on the table near my finished coffee mug. I was yet to recover from the shock I received after going through the page one story with headline Clash of Titans. The report said – The Milky Way galaxy is heading for a collision with the Andromeda galaxy (M31).

Down memory lane I looked back one billion years. In some one of my births I was born in a small planet called the Earth, a member of Solar Family headed by lone yellow star the Sun that lies in the minor spiral arm called the Orion-Cygnus arm at the backwater of the barred spiral galaxy named Milky Way. So I always felt hidden infatuation for this galaxy – my mother galaxy one time.

Stellar collision may be an extreme rarity as the stars are so far apart in comparison to their sizes but galaxy collisions are quite common in our universe. In fact galaxies are unevenly distributed throughout the universe and the gravity makes them clump together into groups. Milky way belongs to a such group called Local Group containing a giant spiral Andromeda, irregular type Large Magellanic Cloud (LMC), Small Magellanic Cloud (SMC) along with a bunch of dwarf galaxies altogether some around thirty plus.

Galaxy clusters typically contain hundreds or even thousands of galaxies. So it is quite possible for galaxies to collide, as many of them share relatively small area of space. How many galaxies are there? Less than a million or more than a billion – nobody still knows. One may safely think that most of the galaxies have definitely undergone several collisions or tidal interactions during their lifetime which may have resulted in the peculiarities in some of their structures.

In galaxy evolution collisions between galaxies are common. Due to weak distribution of matter in galaxies there is in fact no direct collision like a head-on collision between two cars running in opposite direction on same track. If two galaxies pass close to each other, due to action of each one’s gravity on other, their galactic tide can cause both galaxies to deform. Thus one galaxy’s gravity disturbing another galaxy is usually called interaction or tidal interaction. A major form of interaction may be called a collision.

The galactic tide is the tidal force experienced by objects subject to the gravitational field of a galaxy. Such tidal force causes a body in close proximity to stretch out and thus to be deformed. The closer an object is to another the more intensely it is affected by the gravity of other object as the gravitational attraction increases with decreasing separation between objects. This is not only true for a body as a whole it is true for different parts of a body as well. When a galaxy comes in close proximity with another the gravity of one pulls away the surface of the other from its core, causing it to inflate in the direction of approaching galaxy. Depending on the size and mass the shape of each galaxy accordingly changes.


Spiral Galaxies in collision

Above picture shows two interacting galaxies that are being distorted due to gravitational interaction between them.

A satellite galaxy disturbing the primary galaxy’s spiral arms is an example of minor interaction. Sometimes when the interacting galaxies may not have enough momentum to continue travelling after collision, they fall back on each other leading to amalgamation forming a single galaxy. The process of merging of a large galaxy with its companion through tidal gravitational interaction is referred sometimes as Galactic Cannibalism.

Most commonly the resulting galaxy formed due to the gravitational merger of two or more galaxies is an irregular galaxy although elliptical galaxies may also result. During one of my earthly lives in twenty-twenty first century as per the popular calendar system in vogue at that time I read that a galactic cannibalism was occurring between the Milky Way and the Large and Small Magellanic Clouds and also a minor interaction was taking place between Milky Way and Sagittarius Dwarf Galaxy.

Galactic interactions or collisions frequently lead to a burst of new star formation. The giant clouds of gas and dust that are relatively thin and of low density may be compressed due to collision and become increasingly denser as a result of undergoing gravitational collapse, which leads to formation of new generations of stars where normal star formation may have ceased long back.

It is believed that in galactic evolution a galaxy has a strong influence of its environment i.e. other galaxies of its group. It is thought that most galaxies in this universe experience several collisions or tidal interactions over course of their lifetime that resulted in altering their structures, accelerating their evolution as well as induced star formation, building up of heavy elements in the universe, formation of planetary systems and production and distribution of galactic cosmic rays.

Tremendous amount of energy is associated with galactic collision. If two galaxies with masses of the order of 1012 solar masses or 2 x 1042 kilogram meet with a typical relative velocity of 300 kilometres per second the collision energy will be of the order of 1053 Joules. How much is this energy? Well this is equivalent to about total energy involved with 108 – 109 numbers of supernovae!!!

Even with so much huge energy involvement the galaxy collision is not considered as a high-energy phenomenon in usual collision physics sense because of modest encounter velocity of galaxies that is about 0.1% the speed of light. Yet this energy has strong influence in galactic evolution.

We never expect to observe any live galaxy collision like supernova as it is extremely slow by terrestrial standard as the timescale is of the order of 300 million years. Only we can speculate and interpret the phenomena from the various snap-shots taken at random times. Based on theory and observational data astrophysicists make computer simulations which can predict some outcome but nobody can give any guaranty how far it is from reality.

Now that the Andromeda and Milky Way are going to collide within 2 billion years and they are expected to merge to form an elliptical galaxy. What will happen to my solar system? The planet earth will long back be gulped by the sun, which will be in red giant stage then. Of course I doubt whether terrestrial life still exists on earth as by 1 billion years the sun has become enough hot in course of its evolution to destroy all life form on earth. Will sun face a violent collision with any other star of Milky Way or of Andromeda?  Very unlikely. One of the interesting aspects in galaxy collision is the fact that matter in the galaxy are so sparsely distributed that there is almost no chance for any matter-matter collision. The cross-section of a star like sun is about 1017 m2, while the surface density of stars near the sun is of the order of 10 per light year squared i.e. 10-32 per metre squared and thus the collision chance becomes 1 in 1015 for a typical star. Yet prediction says there is a 50% chance that the Solar System will be swept out three times further from the galactic core.

Yet there is some uncertainty. Whether Andromeda will directly collide with Milky Way or make a near miss making some tidal interaction is a big question.