Galaxy Mergers: From Encounter to Coalescence

Galaxy merger: ARP 148 — The Arp 148 galaxy pair (Mayall's Object) imaged by the Hubble Space Telescope (2008). Arp 148 is the stunning result of a collision between two galaxies, creating a ring galaxy and a long-tailed companion galaxy. The collision between the two parent galaxies produced a shock wave that first pulled matter toward the center and then spread it outward, forming a ring. Image source: NASA (new window), ESA (new window), the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration, and A. Evans (University of Virginia, Charlottesville/NRAO/Stony Brook University).

Galaxy Mergers

Galaxy mergers are among the most spectacular and fundamental phenomena in cosmology. Contrary to the static image one might have of the Universe, galaxies are not fixed entities but constantly evolve, drawing closer and merging under the influence of gravitational force. The total duration of a complete merger can range from 0.5 to 2 billion years, depending on the mass of the galaxies and their trajectories. In galaxy clusters, these events contribute to the hierarchical formation of large-scale structures.

A galaxy merger (dynamic process where two galaxies approach and merge) is a major cosmic event that alters morphology, triggers starbursts (star formation from gas clouds), and can fuel central black holes. Fritz Zwicky (1898-1974) was one of the first to study these interactions and their role in galactic evolution.

Stages of Galaxy Mergers

When two galaxies come close enough, their mutual attraction initiates a complex cosmic dance. The gravitational force between two masses is expressed by Newton's law of universal gravitation: \(F = G \frac{m_1 m_2}{r^2}\), where \(G\) is the gravitational constant, \(m_1\) and \(m_2\) are the masses of the galaxies, and \(r\) is the distance between them.

1. Initial Encounter

The first phase begins when two galaxies enter each other's gravitational sphere of influence. At this stage, tidal forces begin to distort their structure, creating perturbations that will gradually amplify.

2. Distortion and Formation of Tidal Tails

Under the influence of tidal forces, galaxies develop characteristic elongated structures: tidal tails and bridges. These spectacular extensions can stretch tens of thousands of light-years and are composed of stars, gas, and dust torn from the parent galaxies. The famous example of the Mice Galaxies (NGC 4676) perfectly illustrates this phenomenon.

3. Coalescence Phase

After several close passes, galaxies lose kinetic energy through dynamical friction and eventually merge into a single entity. The resulting galactic nucleus undergoes violent relaxation, a process that redistributes energy among the stars to reach a new equilibrium.

galaxy merger, the Antennae — A few hundred million years ago, the two spiral galaxies of the Antennae NGC 4038 and 4039 began to collide. The yellow spots to the left and right of the center of the image are the two nuclei of the original galaxies. Image source: NASA/HST (new window),

Astrophysical Consequences of Mergers

Triggered Star Formation

Galaxy mergers compress molecular gas clouds, often triggering intense episodes of star formation called "starbursts." The star formation rate can increase by a factor of 100 or more for a few tens of millions of years.

Activation of Active Galactic Nuclei

Gravitational disturbances can channel gas toward the galactic center, feeding the supermassive black hole and activating an AGN (active Galactic Nucleus). This phenomenon explains the observed correlation between mergers and quasar activity.

Morphological Transformation

Mergers radically transform the morphology of galaxies. The merger of two gas-rich spiral galaxies generally gives rise to an elliptical galaxy, as predicted by Alar Toomre (1940-) in his pioneering work with his brother Juri Toomre (1937-).

Table of Galaxy Mergers

Physical and Morphological Effects of Galaxy Mergers
Type of Merger	Main Effects	Typical Duration	Consequences	Examples
Spiral galaxies of comparable mass	Formation of tidal tails, starbursts, disk deformation	1–2 Gyr	Formation of a massive elliptical galaxy	NGC 4038/4039 (Antennae Galaxies)
Massive galaxy + dwarf galaxy	Minor disturbance, formation of arms and rings	0.5–1 Gyr	Increase in mass and halo reorganization	Milky Way + Sagittarius Dwarf Galaxy
Multiple mergers in a cluster	Progressive accretion, complex gravitational interactions	2–3 Gyr	Formation of giant central galaxies (BCG) and redistribution of dark matter	Abell 2029 (cluster core)

Sources: NASA/IPAC Extragalactic Database (new window), European Southern Observatory (ESO) (new window).