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Interaction Reactions: How Galaxy Encounters Influence Star Formation

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galaxy merger
Once two separate galaxies, galaxy merger NGC 2623 is now a twisted collection of gas and stars extending tens of thousands of light-years across. [ESA/Hubble & NASA; CC BY 4.0]

Over the course of its lifetime, the Milky Way has encountered many other galaxies — from quick flybys to full on collisions, each interaction is important to our galaxy’s evolution. A series of simulations has predicted how galaxy interactions have contributed to star formation across the lifetimes of Milky Way–mass galaxies. 

Merging Galaxies

As galaxies move through the universe, they interact and merge with other galaxies. Sometimes these interactions are quick flybys, leaving behind little evidence of the encounter. Other times, galaxies smash into each other, creating extended tails and messy morphologies. From observations and simulations, scientists have suggested that the merging of galaxies can also trigger increased star formation. However, it is unclear if this behavior is consistent for galaxies across cosmic time. 

In the local universe, observations have shown increased star formation rates for galaxies with nearby neighbors and lower star formation rates for galaxies that are more isolated, likely experiencing very few interactions and mergers. Earlier in the universe, mergers were more common as galaxies were closer together; however, directly observing evidence of galaxy mergers from the distant past is difficult, leading to an incomplete understanding of how ancient mergers impacted star formation rates. Fortunately, simulations can explore where our telescopes can’t quite reach, allowing scientists to track galaxy interactions over time. 

Galaxy Interaction Simulations

Simulation snapshot showing the gas (left) and stellar (right) surface density maps of a major merger event. Click to enlarge. [Li et al 2025]

In order to explore the impact galaxy mergers and interactions have on the star formation activity in Milky Way–mass galaxies, Fei Li (University of Toronto) and collaborators performed simulations using the Feedback in Realistic Environments (FIRE) cosmological simulation suite. In these simulations, the authors track major, minor, and mini interactions that a Milky Way–mass galaxy experiences over ~12 billion years. Over this time, the galaxy’s interaction history and star formation rate is monitored, allowing the authors to explore how interactions may play a role in the galaxy’s star formation. 

torque star formation rate comparison

Torque (blue) and star formation rate (orange) measurements during major merger interaction snapshots from the simulations ran in this study. Not all peaks in star formation rate correspond to peaks in torque, and not all peaks in torque cause a peak in star formation rate. Click to enlarge. [Li et al 2025]

The simulations show that in major interactions, where the interacting galaxies are closer in mass, there is a positive correlation between the torque exerted by the companion and the star formation rate — increased torque often causes a burst of star formation. However, the star formation history in the galaxy shows multiple starbursts, and most often, these starbursts are unrelated to any sort of merger or interaction with another galaxy. In the minor and mini interactions, there is no significant relationship between the torque and star formation rate for the galaxy. While major mergers can trigger some starburst activity, the overall pattern in star formation for Milky Way–mass galaxies is independent of interaction history — less stellar mass created early on can be directly attributed to mergers than previously assumed.

Observation and Simulation Comparison

How do the authors’ results compare to other simulations and observations that explore interaction impacts? Observations that compare the star formation activity of merging and non-merging galaxies find no significant difference between the star formation activity in these two groups of galaxies across the last ~12 billion years, which is consistent with the results of this simulation. Other simulations predict that massive galaxies like the Milky Way go from bursty to steady star formation across cosmic time, and that mergers and interactions do not leave a significant mark on this overall trend of star formation activity. 

This study finds that Milky Way-mass galaxies experience most major interactions during their early years, experience only a few major mergers, and their star formation activity across cosmic time is predominantly driven by internal dynamics within the galaxy itself. These findings provide predictions for what we may observe as advanced telescopes continue to peer into the distant past.

Citation

“The Effect of Galaxy Interactions on Starbursts in Milky Way-mass Galaxies in FIRE Simulations,” Fei Li et al 2025 ApJ 979 7. doi:10.3847/1538-4357/ad94ef