How interacting galaxies form larger structures and trigger star formation and AGN activity
Galaxy collisions and mergers are among the most dramatic events shaping the cosmic landscape. They are not just rare curiosities — these interactions are essential parts of hierarchical structure formation, showing how small galaxies merge into ever larger ones over cosmic history. Beyond mass assembly, collisions and mergers deeply influence galaxy morphology, star formation rates, and central black hole growth, while also playing a crucial role in galaxy evolution. This article reviews the dynamics of galaxy interactions, characteristic observational signatures, and the broad impact on star formation, active galactic nuclei (AGN), and the formation of large-scale structures (groups, clusters).
1. Why galaxy collisions and mergers matter
1.1 Hierarchical assembly in ΛCDM cosmology
In the ΛCDM model, galaxy halos form from small density fluctuations and later merge into larger halos, bringing along the galaxies embedded within them. Because of this:
- Dwarf galaxies → Spiral → Massive elliptical,
- Groups merge → Clusters → superclusters.
These gravitational processes have been occurring since the early epochs of the Universe, gradually weaving the cosmic web. A key part of this picture is how the galaxies themselves merge, sometimes gently, sometimes turbulently, creating new structures.
1.2 Transformative impact on galaxies
Mergers can significantly alter both the internal and external properties of the interacting galaxies:
- Morphological change: Two merging spiral galaxies can lose their disk structures and become elliptical.
- Star formation triggering: Collisions often drive gas to the center, causing intense “starburst” star formation.
- AGN feeding: The same flows can feed central supermassive black holes, igniting quasar or Seyfert-type AGN phases.
- Material redistribution: Tidal tails, bridges, and stellar streams show how stars and gas are thrown around during collisions.
2. Dynamics of galaxy interactions
2.1 Tidal forces and torques
When two galaxies approach, differential gravity causes tidal forces in their stellar disks and gas. This can:
- Stretch galaxies, forming long tidal tails or arcs,
- Form bridges (bridges) of stars and gas connecting both galaxies,
- Remove some gas angular momentum by pushing it toward the center.
2.2 Collision parameters: orbits and mass ratios
The outcome of a collision strongly depends on the orbit geometry and the mass ratio of the interacting galaxies:
- Major merger: When galaxies are similar in size, the result can be a completely remodeled system—often a giant elliptical—accompanied by a powerful star-forming core.
- Minor merger: One galaxy is significantly larger. The smaller one may be disrupted (forming stellar streams) or remain as a satellite that eventually merges with the host.
2.3 Interaction phases
Galaxy mergers last hundreds of millions of years:
- First approach: Tidal features appear, gas is disturbed.
- Multiple passages: With repeated approaches, torques strengthen, triggering more powerful star formation.
- Final remnant: Galaxies merge into a new single system, often becoming more spherical in shape if the merger was a major [1].
3. Signs of mergers
3.1 Tidal tails, bar shapes, and bridges
Impressive structures common in interactions:
- Tidal tails: Long streams of stars and gas extending from a galaxy, often with young star clusters.
- Shells/waves: In elliptical galaxies, remnants of smaller satellite mergers appear as shell-shaped arc features.
- Bridges: Narrow bands of stars or gas connecting two approaching galaxies — indicating an active or past close encounter.
3.2 Star formation "bursts" and enhanced IR emission
In merging galaxies, the star formation rate can increase 10–100 times compared to non-interacting galaxies. Such starbursts cause:
- Bright Hα emission, or if the nucleus is heavily dusty,
- Strong IR radiation: Dust clouds heated by massive young stars glow in the infrared, so such systems become LIRG or ULIRG [2].
3.3 AGN/quasar activity and merger morphology
Gas accretion onto a supermassive black hole can manifest through:
- Bright nucleus: Signs of a quasar or Seyfert galaxy (distinct broad lines, powerful outflows).
- Disturbed outer regions: Prominent structural asymmetries, tidal features — e.g., a quasar host shows signs of a merger or its remnants.
4. Star formation bursts due to gas flows
4.1 Gas transport toward the center
During a close passage, gravitational torques change the angular momentum, forcing molecular gas to fall into the central kiloparsecs. The high-density gas accumulation in the center causes a star formation "burst" — massive new stars form at a much faster rate than in typical spiral galaxies.
4.2 Self-regulation and feedback
Star formation bursts usually last a short time. Stellar winds, supernovae, and AGN outflows can remove or heat the remaining gas, quenching further star formation. Thus, during a merger, a galaxy can become gas-poor, a quiescent elliptical, if the gas was expelled or consumed [3].
4.3 Multi-wavelength observations
Telescopes like ALMA (submillimeter range), Spitzer, or JWST (infrared) and ground-based spectrographs allow tracking cold molecular gas reservoirs, dust emission, and star formation tracers — explaining how mergers control star formation on scales of several kiloparsecs.
5. AGN excitation and black hole growth
5.1 Central "engine" fueling
Many spirals have central black holes, but quasar luminosity requires abundant gas flows to "feed" them near the Eddington limit. Major mergers often cause this:
- Accretion channels: Gas loses angular momentum and accumulates in the nucleus.
- Black hole feeding: This ignites an AGN or quasar, sometimes visible at cosmological distances.
5.2 AGN-driven feedback
An intensely accreting black hole can blow out or heat gas via radiation, winds, or relativistic jets, thereby quenching star formation:
- Quasar mode: High-power episodes with strong outflows, often associated with major mergers.
- "Maintenance" mode: Weaker AGN activity after a starburst can prevent gas from cooling, maintaining a "red and dead" state in the remaining object [4].
5.3 Observational evidence
Some of the brightest AGN or quasars, both local and in the distant Universe, show merger morphology signs — tidal tails, double nuclei, or irregular isophotes — indicating that black hole feeding and mergers often go hand in hand [5].
6. Major and minor mergers
6.1 Major mergers: elliptical formation
When two similarly sized galaxies collide:
- Violent relaxation disrupts stellar orbits.
- Nuclear bulge formation or entire disk disruption can result in a large elliptical or lenticular galaxy.
- Star formation and quasar or AGN mode peak.
Examples like NGC 7252 ("Atoms for Peace") or the Antennae galaxies (NGC 4038/4039) show how currently "colliding" spirals will evolve into a future elliptical [6].
6.2 Minor mergers: gradual growth
When a small galaxy merges with a much larger one:
- Additional massive galaxy halo or nucleus,
- Causes a moderate increase in star formation,
- Leaves morphological signatures, e.g., stellar streams (like Sgr dSph in the Milky Way).
Repeated minor mergers over cosmic time can significantly increase a galaxy's stellar halo and central mass without completely disrupting the disk.
7. Mergers in the Broader Cosmic Environment
7.1 Merger Rate in Cosmic History
Observations and simulations show that the merger rate was highest when the redshift z ≈ 1–3, as galaxies were more densely clustered and thus interacted more frequently. This period also featured the highest cosmic star formation and AGN activity peaks, highlighting the link between hierarchical assembly and intense gas consumption [7].
7.2 In Groups and Clusters
In groups, where galaxy velocities are not very high, collisions are quite frequent. In clusters, where galaxy velocities are higher, direct mergers are rarer but still possible, especially near cluster centers. Over billions of years, continuous mergers form BCG (Brightest Cluster Galaxies), often cD-type ellipticals with very large halos formed from many smaller galaxies.
7.3 The Future Milky Way–Andromeda Merger
Our Milky Way will one day merge with the Andromeda Galaxy (M31) after several billion years. Such a major merger, sometimes called "Milkomeda," will likely create a large elliptical or lenticular system. This shows that collisions are not just a distant phenomenon but also the predicted fate of our galaxy [8].
8. Key Theoretical and Observational Achievements
8.1 Early Models: Toomre & Toomre
The seminal work — Alar and Juri Toomre (1972) proposed simple gravitational simulations demonstrating how disk galaxies form tidal tails during collisions. This helped prove that many "peculiar" galaxies are actually merging spirals [9]. This work sparked decades of research on merger dynamics and morphological outcomes.
8.2 Modern Hydrodynamical Simulations
Current high-resolution simulations (e.g., Illustris, EAGLE, FIRE) study galaxy mergers in the full cosmological context, including gas physics, star formation, and feedback. These models show:
- Starburst intensities,
- AGN fueling modes,
- Final morphological expressions (e.g., elliptical remnants).
8.3 Observations of High Redshift Interactions
Extensive data from "Hubble", JWST, and ground-based telescopes show that mergers and interactions in the early Universe occurred even more actively, driving rapid mass accretion in the first massive galaxies. By comparing observations with theories, astronomers are clarifying how some of the largest elliptical galaxies and quasars formed in early epochs.
9. Conclusion
From minor tidal disturbances to major cataclysms, galaxy collisions are a fundamental factor in cosmic growth and evolution. These collisions change the participants — triggering spectacular bursts of star formation, igniting powerful AGN, and ultimately leading to new morphological forms. They are not random events but organically integrated into the hierarchical formation of Universe structures, where small halos merge into larger ones, and galaxies — along with them.
Such collisions not only transform individual galaxies but also help connect larger structures: forming clusters, creating the cosmic web, contributing to the grand picture of the Universe's structure. As our instruments and simulations improve, we understand these interactions even more deeply — confirming that collisions and mergers, far from being rare occurrences, are indeed the epicenter of galaxy growth and cosmic evolution.
References and further reading
- Barnes, J. E., & Hernquist, L. (1992). "Dynamics of Interacting Galaxies." Annual Review of Astronomy and Astrophysics, 30, 705–742.
- Sanders, D. B., & Mirabel, I. F. (1996). "Luminous Infrared Galaxies." Annual Review of Astronomy and Astrophysics, 34, 749–792.
- Hopkins, P. F., et al. (2006). "A Unified Model for the Co-Evolution of Galaxies and Their Central Black Holes." The Astrophysical Journal Supplement Series, 163, 1–49.
- Di Matteo, T., Springel, V., & Hernquist, L. (2005). "Energy input from quasars regulates the growth and activity of black holes and their host galaxies." Nature, 433, 604–607.
- Treister, E., et al. (2012). "Major Galaxy Mergers Only Trigger the Most Luminous Active Galactic Nuclei." The Astrophysical Journal, 758, L39.
- Toomre, A., & Toomre, J. (1972). "Galactic Bridges and Tails." The Astrophysical Journal, 178, 623–666.
- Lotz, J. M., et al. (2011). "Major Galaxy Mergers at z < 1.5: Mass, SFR, and AGN Activity in Merging Systems." The Astrophysical Journal, 742, 103.
- Cox, T. J., et al. (2008). "The Collision Between the Milky Way and Andromeda." The Astrophysical Journal Letters, 686, L105–L108.
- Schweizer, F. (1998). "Galactic Mergers: Facts and Fancy." SaAS FeS, 11, 105–120.
- Vogelsberger, M., et al. (2014). "Introducing the Illustris Project: Simulating the coevolution of dark and visible matter in the Universe." Monthly Notices of the Royal Astronomical Society, 444, 1518–1547.