galaxy merger

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A galaxy merger is a process of gravitational interaction in which two or more galaxies collide and rearrange into a new system. During the merger, the structure, stellar composition, and star formation rates change. Major mergers (of equal masses) and minor mergers (absorption of a dwarf galaxy) are distinguished. Such interactions are the most important mechanism of galaxy evolution in hierarchical cosmology, where small objects gradually assemble into larger ones.

History

The idea that galaxies could merge arose after Edwin Hubble proved in the 1920s that many nebulae are other galaxies. Later, astronomers noticed distorted galaxies with 'tails' and double nuclei, which were explained as merger remnants.

How it works

Two galaxies attract each other by gravity. They slowly approach, pass through one another, stretching tidal tails of stars and gas, then come together again and eventually mix into one entity — much like two whirlpools merging into one large vortex.

💡 When two spiral galaxies merge, an elliptical galaxy is often born, and the central supermassive black holes also merge, emitting powerful gravitational waves.

A galaxy merger is a long (100 million to 1 billion years) dynamical process during which dark matter halos, stellar, and gas components of two galaxies mix and relax toward equilibrium. Gravitational instability unites them into a single system; the gas experiences compression and turbulence, triggering a burst of star formation and nuclear activity (accretion onto black holes). They are modeled numerically with dark matter and feedback. At the final stage, an elliptical galaxy forms with depleted gas and an old stellar population.

History

In the 1920s, Edwin Hubble established the extragalactic nature of nebulae and created a galaxy classification, later supplemented by the idea of collisions. Fritz Zwicky in the 1930s, studying galaxy clusters, noted interactions and the need for dark matter to hold systems together. In the 1970s, Vera Rubin obtained galaxy rotation curves, proving the dominance of dark halos, which became the foundation for modern understanding of merger dynamics. Modern supercomputer simulations like Illustris trace mergers from the early Universe to the present day.

How it works

As they approach, two galaxies experience gravitational interaction according to the law of universal gravitation. Tidal forces pull long tails from them, dynamical friction slows their orbital motion, and the components merge after several passes. Gas, unlike stars, undergoes hydrodynamic shocks, cools, and compresses, forming dense clouds from which new stars are born. Central supermassive black holes approach each other to distances where gravitational wave emission (per general relativity) dominates and they coalesce. The process is described by equations of motion of matter in an expanding Universe, with dark matter playing a key role via the gravitational potential of halos.

M_J \approx \frac{3 k_B T r}{2 G \mu m_H}

M_J — Jeans mass, k_B — Boltzmann constant (k_B ≈ 1.381×10⁻²³ J/K), T — cloud temperature, r — cloud radius, G — gravitational constant (G ≈ 6.674×10⁻¹¹ m³/(kg·s²)), μ — mean molecular weight (in atomic units, for molecular hydrogen μ ≈ 2), m_H — mass of hydrogen atom (m_H ≈ 1.674×10⁻²⁷ kg)

Open problems & fun facts

How exactly does feedback from supermassive black holes regulate star formation during mergers?; A complete explanation of elliptical galaxy formation: why do they lose the ability to form stars?; Prediction of the gravitational wave background characteristics from mergers of all supermassive black holes.

💡 During a merger, a ring galaxy can form — like the famous Cartwheel Galaxy — when one galaxy passes exactly through the center of another.

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galaxy merger

History

How it works

History

How it works

History

How it works

Open problems & fun facts

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