What Are Galaxy Mergers - Galactic Interactions and Collisions?

Galaxy mergers occur when two galaxies collide and are the most violent of galaxy interactions. The result is the creation of new galaxies.

Galaxies, the largest single objects in the universe, are truly awe-inspiring in their scale. Each one contains upwards of trillions of stars in a single gravitationally bound system. While the universe is extremely large, and many galaxies are very far apart, it is common for galaxies to group in clusters. It's also common for them to collide with each other, a process that leads to the creation of new galaxies, a phenomenon that never fails to inspire wonder.

Large galaxies, like the Milky Way and Andromeda Galaxy, formed as smaller objects collided and merged. Today, astronomers observe smaller satellites orbiting near the Milky Way and Andromeda. These 'dwarf galaxies' have some characteristics of larger galaxies but are on a much smaller scale and can be irregularly shaped. Our galaxy is 'cannibalizing' some of its companions, which means it is absorbing or incorporating them into its structure.

The Milky Way's largest satellites are called the Large and Small Magellanic Clouds. These are not actual clouds but rather two irregular dwarf galaxies that are orbiting our galaxy in a billions-of-year orbit and may never merge with the Milky Way.

Large-galaxy collisions, an ongoing process in the universe, create huge new galaxies in the process. Often, two large spiral galaxies will merge, and due to the gravitational warping that precedes the collision, the galaxies will lose their spiral structure. Once the galaxies are merged, astronomers suspect that they form a new structure known as an elliptical galaxy. Occasionally, depending on the relative sizes of the merging galaxies, an irregular or peculiar galaxy results from the merger. This ongoing process of galactic mergers keeps the universe constantly changing and evolving, a topic of endless fascination for astronomers.

While galaxies themselves may merge, the process doesn't always hurt the stars they contain. This is because while galaxies have stars and planets, there's a lot of empty space and giant clouds of gas and dust. However, colliding galaxies that do contain a large amount of gas enter a period of rapid star formation. This intense burst of star formation, usually much greater than a non-colliding galaxy's average star formation rate, creates what astronomers call a 'starburst galaxy '.

Merger Between the Milky Way and Andromeda

The Milky Way and Andromeda are due to merge in about 2 billion years from now. The galaxies will not collide in an explosive collision but will merge over tens of millions of years. Due to the large distance between stars in a galaxy, there is unlikely to be any collisions. There will, however, be massive gravitational disruption, severely disrupting not only the spiral arms' structure but also the whole galaxy itself. Initially, the stars on the leading edges will be drawn together, elongating the galaxies somewhat. As the merger advances, the galaxies will appear to flow through each other. As they pass through each other, their velocities will slow and enter into an orbit around each other as their momentum slows. The resultant merger will form a giant elliptical galaxy once the system has become stable. First, the stars on the leading edges will be drawn together, elongating the galaxies somewhat. As the merger continues, the galaxies will appear to flow through each other. As they pass through each other, their velocities will slow and enter into an orbit around each other as their momentum slows. The resultant merger will form a giant elliptical galaxy once the system has become stable.

Dark matter constitutes 95% of the mass of a galaxy, mainly in the halo. In a galactic merger, the interaction of dark matter is not widely known. It is, however, suspected that dark matter from each galaxy passes through each other with little to no interaction. They are said to be weakly interacting.

Every galaxy is suspected to have a supermassive black hole at its centre. During the galaxy merger, they will be drawn towards each other. As this happens, their massive amount of energy will be transferred to the surrounding stars, causing them to enter into a slingshot-style orbit. As the black holes get closer, they emit gravitational waves, transferring more orbit energy to the surrounding stars. Eventually, the two black holes will merge, forming an active galactic nucleus or quasar.

The Sun, a part of the Milky Way, is expected to be propelled towards the galaxy's centre as a result of the merger with Andromeda. This may tear the Sun apart by the new supermassive black hole, or equally probable; the Sun could be flung out of the galaxy completely when it goes on a slingshot around the black hole. The possible effects of the merger of the Milky Way into Andromeda has become a source of high anticipation and curiosity for professional and non-professional astronomers alike.