Editorial Feature

Everything to Know About When Galaxies Collide

Galaxies are vast collections of stars, gas, and dark matter in constant motion, influenced by the gravitational forces that bind them together. When two galaxies come into close proximity, their gravitational interactions can lead to a galactic collision. These galactic collisions are slow processes that unfold over cosmic timescales, typically spanning hundreds of millions of years. This article discusses the phenomenon of galactic collision, the tools used, the nature of evidence, and recent relevant research.

When Galaxies Collide, what happens when galaxies collide

Image Credit: Wolfgang Kloehr/Shutterstock.com

What happens when Galaxies Collide?

When galaxies collide, one significant phenomenon that occurs is dynamical friction, also known as a Chandrasekhar friction. In astronomy, the process by which a large object traveling through a dense medium like galaxies undergoes a slowing because of interactions with the surrounding particles or objects is known as dynamical friction.

The strength of dynamical friction depends on various factors, including the mass of the moving object, the density and distribution of the surrounding particles, and their relative velocities. In dense environments, such as the cores of galaxy clusters, dynamical friction can be a significant factor in shaping the distribution and dynamics of celestial objects over long periods of time.

Tools to Observe Galactic Collisions

While the timescales of galactic collisions are far beyond the scope of a human lifetime, astronomers have developed sophisticated tools and techniques to observe and study these events. For instance, telescopes equipped with advanced imaging capabilities allow astronomers to capture images of colliding galaxies at various stages of their interaction. The Hubble Space Telescope, in this regard, has been very helpful in providing valuable insights into the morphological changes and structural transformations that occur during these cosmic encounters. Similarly, ground-based observatories equipped with adaptive optics have also contributed in this regard by reducing the blurring effects of Earth's atmosphere, enabling clearer observations.

In addition to optical telescopes, radio telescopes can detect emissions from neutral hydrogen gas, offering a unique perspective on the distribution and dynamics of the gas clouds involved in the collision. Similarly, X-ray and infrared observations conducted with space-based telescopes like Chandra and Spitzer reveal the energetic processes associated with star formation and the activity of supermassive black holes.

Evidence of Galactic Collisions

The evidence supporting the occurrence of galactic mergers is not limited to the images captured by telescopes. Various observational signatures across the electromagnetic spectrum provide a comprehensive understanding of these cosmic phenomena. For instance, elongated trails of stars, known as tidal trails, observable in optical and infrared wavelengths, provide direct evidence of the gravitational interactions involved in galactic collisions.

Similarly, another footprint of galactic collisions is the collision of gas clouds within galaxies, generating shockwaves, heating the gas, and triggering intense star formation. Additional evidence of galaxy collisions is provided by detecting gravitational waves produced by the merger of black holes using instruments like LIGO and Virgo.

Recent Studies on Galactic Collisions

Numerical Simulations and DMDG Formation

In a recent study published in The Astrophysical Journal, researchers investigated the formation of dark matter-deficient galaxies (DMDGs) through high-velocity collisions between gas-rich, dwarf-sized galaxies. The study utilized high-resolution numerical simulations with mesh-based and particle-based codes to explore the possibility of DMDG formation during such collisions.

The simulations demonstrated that DMDGs can indeed form as a result of high-velocity galaxy collisions, where the collision separates dark matter from warm disk gas. The warm disk gas is then compressed by shock and tidal interactions, leading to the formation of stars. The researchers also explored a large simulated universe, IllustrisTNG, and identified high-velocity galaxy collision events where DMDGs could be expected. However, the study emphasized the importance of numerical resolution in accurately capturing the collision-induced DMDG formation scenario. This research sheds light on unconventional pathways for galaxy formation, challenging existing models.

Dynamical Structure of DMDGs

In another recent study, researchers conducted galaxy collision simulations using the adaptive mesh refinement code Enzo with improved 1.25 pc resolution, shedding light on the mysterious formation of dark matter deficient galaxies such as NGC1052-DF2 and NGC1052-DF4. The simulations demonstrated that high-velocity galaxy collisions, particularly with a relative velocity of ∼300 km s−1, induce the formation of DMDGs and their star clusters (SCs) simultaneously.

The study, surpassing previous resolution limitations, revealed the dynamical structure of resulting DMDGs and their SCs, offering insights into the formation history of these galaxies. The findings suggest that DMDGs and their luminous SCs could form concurrently through high-velocity galaxy collisions, aligning with key properties observed in NGC1052-DF2 and NGC1052-DF4. This research contributes to understanding the enigmatic nature of galaxies lacking dark matter and exhibiting unusually bright globular clusters.


In conclusion, galactic collisions are intricate processes occurring over cosmic timescales, shaping the universe's dynamic landscape. Astronomers employ advanced tools like the Hubble Space Telescope and radio telescopes to observe and study these events, capturing morphological changes and gas dynamics.

Recent studies utilizing high-resolution simulations investigate the formation of dark matter-deficient galaxies through high-velocity collisions, offering insights into their dynamic structure. With ongoing advancements in observational techniques and new technologies, the study of galactic collisions will unveil even more secrets about the dynamic and ever-changing nature of our vast cosmic landscape.

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Reference and Further Reading

Shin, E. J., Jung, M., Kwon, G., Kim, J. H., Lee, J., Jo, Y., & Oh, B. K. (2020). Dark matter deficient galaxies produced via high-velocity galaxy collisions in high-resolution numerical simulations. The Astrophysical Journal. https://doi.org//10.3847/1538-4357/aba434

Duc, P. A., & Renaud, F. (2013). Tides in colliding galaxies. Lecture Notes in Physics, Berlin Springer Verlag, J. Souchay, S. Mathis, and T. Tokieda. https://doi.org/10.1007/978-3-642-32961-6

Struck, C. (1999). Galaxy collisions. Physics Reports. https://doi.org/10.1016/S0370-1573(99)00030-7

Lee, J., Shin, E. J., & Kim, J. H. (2021). Dark matter deficient galaxies and their member star clusters form simultaneously during high-velocity galaxy collisions in 1.25 pc resolution simulations. The Astrophysical Journal Letters. https://doi.org//10.3847/2041-8213/ac16e0

McKinney, J., et al. (2021) Dust-enshrouded AGNs Can Dominate Host-galaxy-scale Cold Dust Emission. The Astrophysical Journal. https://doi.org/10.3847/1538-4357/ac185f

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Taha Khan

Written by

Taha Khan

Taha graduated from HITEC University Taxila with a Bachelors in Mechanical Engineering. During his studies, he worked on several research projects related to Mechanics of Materials, Machine Design, Heat and Mass Transfer, and Robotics. After graduating, Taha worked as a Research Executive for 2 years at an IT company (Immentia). He has also worked as a freelance content creator at Lancerhop. In the meantime, Taha did his NEBOSH IGC certification and expanded his career opportunities.  


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