The Connection Between Galaxy Collisions and the Formation of New Stars

Galaxies are not static islands in space; they are dynamic systems moving under the influence of gravity. Over cosmic time, galaxies interact, merge, and collide, often triggering episodes of intense star formation. These collisions are among the most energetic events in the universe, reshaping galaxies and seeding the next generations of stars. Understanding the connection between galaxy collisions and star formation is crucial for piecing together how galaxies evolve from the early universe to the present day.

The Physics of Galaxy Collisions

When galaxies collide, their stars rarely hit each other directly because of the vast distances between them. Instead, it is the gravitational forces that dominate the interaction. Dark matter halos, gas, and dust all respond to the changing gravitational field, distorting the shapes of the galaxies and setting the stage for new stellar birth.

Gravitational interactions and tidal forces

As two galaxies approach, tidal forces stretch them, pulling material into long streams known as tidal tails. These tails can extend for hundreds of thousands of light-years, containing gas and stars ripped from the parent galaxies. The gravitational tug-of-war also compresses existing gas clouds, raising their density and temperature. This compression is a key mechanism that can trigger star formation, as dense pockets of gas become gravitationally unstable and collapse to form protostars. The detailed shape of tidal tails encodes the history of the encounter and the distribution of dark matter in the system.

Shock waves in the interstellar medium

When gas-rich galaxies collide at high relative speeds, shock waves propagate through the interstellar medium. These shocks heat the gas, but in dense regions they can also radiate away energy efficiently, leading to rapid cooling and further collapse of gas clouds. Shocks are especially effective at sweeping up and piling up gas into dense filaments and clumps, providing ideal sites for star clusters to form. Observations with radio telescopes reveal that shock-compressed gas in merging systems often has increased molecular line emission, a direct signature of star formation potential.

Gas dynamics and turbulence

Collisions inject vast amounts of kinetic energy into the interstellar medium, driving supersonic turbulence. This turbulence can act in two ways: it creates density fluctuations that seed gravitational collapse, but it also provides pressure support that may delay star formation. The net effect depends on the strength and scale of the turbulence. Numerical simulations show that merger-driven turbulence tends to enhance the star formation rate by an order of magnitude compared to quiescent galaxies by keeping gas clouds from dissipating and by promoting the formation of giant molecular complexes.

Role of dark matter

Dark matter halos play a significant role in galaxy collisions. They contribute the majority of the gravitational mass, funneling the luminous galaxies along collision courses. Simulations show that the dark matter distribution influences the timing and location of gas compression. Without dark matter, collisions would be far less frequent and less disruptive, altering the rate of star formation in the universe. The extended dark matter halos also slow the orbital decay of merging galaxies, determining whether they merge quickly or undergo multiple passages before coalescing.

Triggers for Star Formation

Star formation does not occur spontaneously; it requires dense, cold molecular gas that can collapse under its own gravity. Galaxy collisions provide multiple triggers that push molecular clouds over the edge of gravitational instability.

Compression of molecular clouds

The direct collision of two galaxies forces gas clouds together. This compression increases the density and reduces the Jeans length, making a larger fraction of the cloud’s mass susceptible to collapse. Observations show that merging galaxies often contain supermassive molecular gas complexes that are absent in isolated galaxies. These complexes can spawn hundreds of thousands of stars in a few million years. High-resolution imaging from the Atacama Large Millimeter/submillimeter Array (ALMA) has resolved these complexes into filamentary networks that are actively forming star clusters.

Cloud-cloud collisions

Individual molecular clouds within the colliding galaxies can smash into each other at speeds of tens to hundreds of kilometers per second. Such cloud-cloud collisions are now recognized as a major trigger for forming massive star clusters. The collision creates a thin, shocked layer where gas accumulates and fragments, leading to the rapid formation of very massive stars. This process is invoked to explain the origin of super star clusters—dense aggregates of young stars observed in merging systems like the Antennae Galaxies.

Galactic tidal streams and filaments

After the initial collision, tidal tails and filaments contain a significant amount of gas. This material is often turbulent and compressed, allowing star formation to proceed far from the main galaxy bodies. Examples like the Antennae Galaxies show bright knots of young stars along the tidal tails, demonstrating that star formation can continue long after the initial passage. These tidal features are also laboratories for studying star formation in low-metallicity environments, as the gas stripped from the parent galaxies can have different chemical abundances.

Types of Galaxy Interactions

Not all collisions are equal. The outcome depends on the mass ratio, gas content, and geometry of the encounter. Astronomers classify interactions into several categories.

Major mergers

When two galaxies of similar mass merge, the collision is a major merger. These events are the most violent and can completely transform both galaxies. The resulting system is often an elliptical galaxy, with a burst of star formation (a starburst) during the peak of the interaction. Major mergers are thought to be responsible for the formation of the most massive elliptical galaxies and are linked to the activation of active galactic nuclei (AGN). The star formation rate in a major merger can exceed 100 solar masses per year for a short period, producing a population of massive stars that explode as supernovae within a few million years.

Minor mergers and accretions

In a minor merger, a small satellite galaxy is consumed by a larger host. These events are more common than major mergers. They can trigger star formation in the central regions of the larger galaxy as the satellite’s gas is stripped and funnelled inward. Minor mergers also add stars and gas to the disk, contributing to the build-up of spiral galaxies over time. The frequency of minor mergers means they are likely the dominant mode of star formation enhancement in the local universe, though their individual impact is less dramatic than major mergers.

Galaxy flybys

Not every encounter results in a merger. In a flyby, two galaxies pass close enough to gravitationally perturb each other but remain separate. Even a distant flyby can distort gas distributions and trigger star formation in spiral arms. Many observed “peculiar” galaxies with disturbed shapes are the product of recent flybys that did not result in a merger. Flybys are particularly important for triggering star formation in the outer disks of galaxies, where gas is otherwise stable.

Starburst Galaxies

A starburst galaxy is one that is forming stars at an exceptionally high rate—far above what can be sustained for a long period. Most starbursts are triggered by interactions or mergers. The intense radiation from young massive stars heats the surrounding dust, making starburst galaxies extremely luminous in the infrared. In some cases, the star formation rate in a starburst can exceed 100 solar masses per year, compared to just a few solar masses per year in a normal spiral like the Milky Way.

Ultraluminous Infrared Galaxies (ULIRGs)

The most extreme starbursts are classified as Ultraluminous Infrared Galaxies (ULIRGs). These systems radiate over 1012 solar luminosities in the infrared, powered predominantly by a heavily dust-enshrouded starburst. ULIRGs are almost always the product of major mergers between gas-rich galaxies. The dust that reprocesses the starlight also makes them faint in optical light, but observations with infrared telescopes like the Spitzer Space Telescope and ALMA have revealed their intense activity. Studying ULIRGs provides insight into the peak of cosmic star formation at high redshift, where such systems were common.

Feedback mechanisms

The prodigious star formation in a starburst eventually generates strong feedback in the form of stellar winds, supernova explosions, and radiation pressure. This feedback can expel gas from the galaxy, quenching the starburst after a few tens of millions of years. Studying starburst galaxies therefore provides a window into the rapid evolution of galaxies and the interplay between star formation and feedback. In some cases, feedback can drive galactic-scale outflows that enrich the intergalactic medium with heavy elements, a process known as chemical enrichment.

Observational Examples

Several well-studied systems illustrate the connection between collisions and star formation.

The Antennae Galaxies (NGC 4038/4039)

The Antennae Galaxies are perhaps the most famous pair of colliding galaxies, located about 45 million light-years away in the constellation Corvus. High-resolution images from the Hubble Space Telescope reveal dozens of young star clusters strung along the tidal tails and between the two galactic nuclei. The collision is producing stars at a rate roughly ten times higher than normal. The Antennae serve as a textbook example of how merging triggers widespread star formation. X-ray observations also show hot gas and supernova remnants, evidence of the ongoing feedback from massive stars.

More about the Antennae Galaxies on Wikipedia.

The Mice Galaxies (NGC 4676)

The Mice Galaxies are a pair of interacting spiral galaxies with long tidal tails that resemble mouse tails. The collision has compressed enormous gas clouds, leading to regions of intense star formation, especially at the base of the tails. Observations in visible and infrared light show bright clusters of young stars that have formed within the last few million years. The Mice are in an earlier stage of interaction than the Antennae, offering a snapshot of the process before the nuclei merge.

The Cartwheel Galaxy

The Cartwheel Galaxy is a lenticular galaxy with a distinct ring of star formation, created after a smaller galaxy passed directly through its center. The resulting shock wave swept up gas into a dense ring that is now forming stars at a rapid pace. The Cartwheel demonstrates that even a high-speed penetration (a “bullet” collision) can produce a spectacular starburst ring. The ring’s structure is not static; it expands and fragments over time, eventually producing dozens of giant star-forming regions.

NASA Hubble on the Cartwheel Galaxy.

Arp 220 – A Prototypical ULIRG

Arp 220 is the nearest example of an ultraluminous infrared galaxy and the aftermath of a recent major merger. Its two nuclei are still distinct but enshrouded in dust, and it hosts extreme starburst activity. ALMA observations have revealed massive rotating molecular disks and outflows of gas driven by the starburst and a possible embedded AGN. Arp 220 exemplifies the final stages of a merger-induced starburst and is a crucial laboratory for understanding feedback and the transformation of galaxies.

The Future Milky Way–Andromeda Collision

Our own Milky Way is on a collision course with the Andromeda Galaxy (M31). In about 4.5 billion years, the two galaxies will begin to merge, triggering a burst of star formation. Simulations predict that the combined system will become an elliptical galaxy—sometimes nicknamed “Milkomeda.” The collision will also funnel gas toward the galactic center, possibly fueling a period of AGN activity. While this event is far in the future, it underscores that galaxy collisions are an ongoing and inevitable part of cosmic evolution.

Evidence from Simulations and Observations

Our understanding of collision-induced star formation relies heavily on computer simulations that model the dynamics of gas, stars, dark matter, and feedback. Modern cosmological simulations, such as IllustrisTNG, show that galaxy mergers are a primary driver of the star formation rate density at high redshift (when the universe was 2–4 billion years old). Observations with ALMA have revealed molecular gas in distant merging galaxies, confirming that these environments are rich with the raw material for new stars. Large survey programs like CANDELS and GOODS have identified thousands of merging systems at high redshift, linking the merger rate to cosmic star formation history.

A seminal study by Kormendy & Kennicutt (2010) emphasizes that while galaxy interactions can boost star formation, the relationship is not linear—feedback and gas supply are critical factors. Another key paper by Mihos & Hernquist (1996) used simulations to show that the specific geometry of a merger determines where and when star formation peaks. More recent hydrodynamical simulations, such as the FIRE project, have refined our understanding of how stellar feedback regulates star formation in mergers, demonstrating that the most intense bursts are short-lived and self-limiting.

Implications for Galaxy Evolution and Cosmic History

Galaxy collisions and the resulting star formation have shaped the universe we see today. Cosmic star formation peaked around 10 billion years ago, a time when mergers were much more common. Many of the most massive galaxies in the modern universe contain old stellar populations that likely formed during major mergers. These events also play a role in quenching star formation by heating or ejecting gas, transforming star-forming spiral galaxies into red-and-dead ellipticals.

Moreover, the supermassive black holes at the centers of galaxies can be fed by gas funnelled during mergers, triggering active galactic nuclei (AGN). Feedback from AGN can further regulate star formation, creating a complex feedback loop that astrophysicists continue to investigate. Understanding these processes is essential for building a complete picture of galaxy formation. The coming decade, with missions like the James Webb Space Telescope and the Nancy Grace Roman Space Telescope, will provide deeper views of merging galaxies in the early universe, testing theories of merger-driven evolution.

Conclusion

The connection between galaxy collisions and star formation is one of the cornerstones of modern astrophysics. From the compression of gas clouds to the formation of spectacular starburst galaxies, collisions drive the evolution of galaxies across cosmic time. Observational examples like the Antennae, Cartwheel, and Arp 220 provide vivid snapshots of this process, while simulations and surveys reveal its statistical importance. As telescopes like the James Webb Space Telescope push our view deeper into the early universe, we expect to find even more merging galaxies in the act of forming stars, further cementing the crucial role of collisions in building the universe around us.