Galaxy merger

A galaxy merger is a massive cosmic event where two or more galaxies collide. These interactions are considered the most violent types of movement in the universe. Astronomers study these mergers because they act as a fundamental measurement of galaxy evolution. By observing these collisions, scientists gain clues about how galaxies grew into their current shapes over billions of years. The merger rate helps us understand the long-term history of the cosmos.

Gravity is the primary engine behind these massive collisions. As galaxies approach one another, their gravitational pull begins to affect the stars and dark matter within each system. In many galaxies, stars follow an orderly rotation within flat disks. However, as the merger progresses, the gravitational potential changes very quickly. This causes a process called violent relaxation, where star orbits are greatly altered. The stars lose their original paths and move in a complicated, random network. This transformation of ordered motion into random energy is why many mergers result in elliptical galaxies.

Mergers also act as intense factories for new stars. During a major merger, giant molecular clouds rush toward the center of the colliding systems. When these clouds collide with one another, they condense into new stars. The star formation rate (SFR) during a major merger can reach thousands of solar masses per year. This is a massive increase compared to our own Milky Way, which produces only about two new stars each year. However, this process is also self-limiting. The intense activity uses up most of the available gas, leaving very little for future star formation.

Because they use up so much gas, merged galaxies often enter a phase called the post-starburst (PSB) phase. In this stage, the galaxy has very few young stars left because the gas has been depleted. This is a common characteristic of elliptical galaxies seen in the local universe. Scientists believe many elliptical galaxies are the end products of major mergers that occurred 1 to 10 billion years ago. These ancient mergers were more pronounced because galaxies contained much more gas and molecular clouds back then. A major merger can quench star formation more rapidly than almost any other phenomenon in the universe.

Astronomers classify mergers in several different ways. They can be grouped by the number of galaxies involved, such as a binary merger of two galaxies or a multiple merger of three or more. They can also be classified by size. A minor merger occurs when one galaxy is significantly larger than the others. The larger galaxy often "eats" the smaller one in a process called galactic cannibalism. In contrast, a major merger involves two galaxies of similar size. Our own Milky Way is currently experiencing minor mergers with the Canis Major Dwarf Galaxy and possibly the Magellanic Clouds.

Another way to classify mergers is by their gas richness. A wet merger happens between gas-rich "blue" galaxies and typically triggers massive star formation and quasar activity. A dry merger involves gas-poor "red" galaxies and mainly serves to increase stellar mass without much new star formation. There are also damp mergers, which involve one blue and one red galaxy with enough gas to fuel some star formation. Mixed mergers occur when gas-rich and gas-poor galaxies combine. These different types of collisions lead to very different types of resulting galaxies.

To understand these complex events, scientists use advanced computer simulations. Researchers like Jennifer Lotz have used these tools to interpret images from the Hubble Space Telescope. Her team studied 57 different merger scenarios, looking at different orbits, impact angles, and orientations. Other researchers use deep learning and convolutional neural networks to predict the timescales of these events. For example, simulations can predict how long it takes for galaxies to reach maximum separation or to fully fuse. One famous prediction involves our own neighborhood: the Milky Way and Andromeda are expected to collide in about 4.5 billion years. This major merger will likely transform both spiral galaxies into one giant elliptical galaxy.