Galaxy mergers occur when two or more galaxies collide. This is the most intense type of galactic interaction. Gravitational interactions between galaxies and friction between gas and dust have significant effects on the involved galaxies. However, the exact outcomes of such mergers depend on a wide range of parameters, such as the collision angles, velocities, and relative sizes/compositions, making it a highly active area of research.
Galaxy mergers are important because the merger rate is a fundamental measure of galaxy evolution. They also provide astronomers with clues about how galaxies have grown into their current forms over long periods of time.
Two galaxies in the process of merging. (Credit: NASA, ESA, and the Hubble Team) [1]
During a merger, the stars and dark matter in each galaxy are affected by the approaching galaxy. In the final stages of the merger, the gravitational potential changes so rapidly that the orbits of stars are significantly altered, and they lose traces of their previous paths. This process is called "violent relaxation." For example, when two disk galaxies collide, their stars begin to rotate in the planes of the two separate disks. During the merger, this orderly motion turns into random energy and thermalizes. The resulting galaxy is dominated by stars orbiting in a complex and random network of interactions, similar to what is observed in elliptical galaxies.
Mergers are also sites of intense star formation. The star formation rate (SFR) during a major merger can reach thousands of solar masses per year, depending on the gas content and redshift of each galaxy. Typical merger SFRs are less than 100 solar masses per year, which is still much higher than the Milky Way Galaxy, which produces only a few new stars (approximately 2 stars) each year. Although stars in galaxy mergers rarely come close enough to collide, giant molecular clouds rapidly fall toward the center of the galaxy and collide with other molecular clouds.
These collisions cause the clouds to condense and form new stars. However, this process was more prominent during the mergers that formed most of the elliptical galaxies we see today, which likely occurred 1-10 billion years ago when galaxies had much more gas and molecular clouds. Additionally, shock waves generated by colliding gas clouds far from the galaxy's center can trigger the formation of new stars. As a result of all this activity, galaxies tend to have very little gas left to form new stars after a merger. Therefore, if a galaxy undergoes a major merger and several billion years pass, very few young stars will remain in the galaxy.
This is what we observe in elliptical galaxies today: very little molecular gas and very few young stars. This is thought to be because elliptical galaxies are the end products of major mergers that consumed most of the gas during the merger, effectively quenching further star formation afterward.
The galaxy NGC 3256, located approximately 100 million light-years away, was formed by the merger of two galaxies. (Credit: NASA, ESA, and the Hubble Team) [2]
Categories of Galaxy Mergers
Galaxy mergers are classified into different groups based on the number of merging galaxies, their relative sizes, and their gas richness.
By Number
Mergers can be categorized by the number of galaxies involved:
1. Binary Merger: The merger of two interacting galaxies.
2. Multiple Merger: The merger of three or more galaxies.
By Size
Mergers can be categorized by how much the largest galaxy involved changes in size or shape:
1. Minor Merger: If one galaxy is significantly larger than the others, the merger is considered minor. The larger galaxy typically "consumes" the smaller one, a phenomenon known as "galactic cannibalism," where the larger galaxy absorbs most of the smaller galaxy's gas and stars with little other significant impact. The Milky Way Galaxy, which includes Earth, is thought to be currently consuming several smaller galaxies, such as the Canis Major Dwarf Galaxy and possibly the Magellanic Clouds.
2. Major Merger: The merger of two spiral galaxies of roughly the same size is a major event. If they collide at the right angles and speeds, they will likely merge in a way that expels much of their dust and gas, often involving feedback mechanisms that include active galactic nuclei. This is thought to be the driving force behind many quasars. The result is an elliptical galaxy, and many astronomers assume this is the primary mechanism that creates elliptical galaxies.
By Gas Richness
Mergers can be categorized by the degree of interaction of the gas carried within and around the merging galaxies:
1. Wet Merger: Wet mergers occur between gas-rich galaxies (blue galaxies). These mergers typically produce large amounts of star formation, transform disk galaxies into elliptical galaxies, and trigger quasar activity.
2. Dry Merger: A merger between gas-poor galaxies (red galaxies) is called a dry merger. Dry mergers usually do not significantly alter the star formation rates of galaxies but play an important role in increasing stellar mass.
3. Damp Merger: If there is enough gas between blue and red galaxies to fuel significant star formation but not enough to form globular clusters, a damp merger occurs.
4. Mixed Merger: A mixed merger occurs when gas-rich and gas-poor galaxies merge.
The collision of the Taffy Galaxies. (Credit: International Gemini Observatory) [3]
The predicted merger between the Milky Way Galaxy and the Andromeda Galaxy. An image representing Earth's night sky 3.75 billion years from now. (Credit: NASA, ESA, Z. Levay, R. van der Marel) [4]
