As one galaxy smashed into the other, it pierced its cosmic victim, leaving behind a high level of radiation that will interfere with star formation in what scientists are calling a unique astronomical observation.
This is the first time astronomers have witnessed such an event, which was made possible by the European Southern Observatory’s Very Large Telescope (ESO’s VLT) and the Atacama Large Millimeter/Submillimeter Array (ALMA). The international effort hopes to be the first of many to observe similar ancient, distant clashes between young galaxies in the early universe.
A Battle in the Deep Space
In a remote pocket of the universe, two galaxies entered a dramatic cosmic collision. Their repeated close encounters occurred at speeds of 500 kilometers per second, initially inflicting only minor disruptions. But eventually, one galaxy gained the upper hand thanks to a powerful weapon: a quasar. Acting like a radioactive spear, the quasar pierced its galactic opponent, leaving behind a radiation-laced wound.
“We hence call this system the ‘cosmic joust’,” said study co-lead Pasquier Noterdaeme, a researcher at the Institut d’Astrophysique de Paris and the French-Chilean Laboratory for Astronomy in Chile, referencing the duels of medieval knights.
Witnessing an Ancient Conflict
Quasars are powered by radiation-emitting supermassive black holes at the centers of some galaxies. Both galactic mergers and quasars were more common in the universe’s early epochs. This event occurred 11 billion years ago, when the universe was just 18% of its current age. Due to its great distance, light from the clash is only now reaching Earth.
“Here we see for the first time the effect of a quasar’s radiation directly on the internal structure of the gas in an otherwise regular galaxy,” explained study co-lead Sergei Balashev, a researcher at the Ioffe Institute in St. Petersburg, Russia.
Astronomers observed that the quasar’s radiation disrupted the gas and dust clouds of the losing galaxy. While the smallest and densest regions managed to endure, scientists believe they are too small to sustain star formation. In the aftermath of the quasar’s assault, the galaxy was left with fewer stellar nurseries.
“These mergers are thought to bring huge amounts of gas to supermassive black holes residing in galaxy centres,” Balashev noted.
To make matters worse, the material from the victim galaxy further fueled the black hole, allowing the quasar to grow even more powerful.
Pursuing Galaxy Data and Next Steps
ALMA provided the necessary high-resolution imaging to capture merging galaxies across such vast distances. ESO’s VLT, equipped with the X-shooter instrument—a multi-wavelength, medium-resolution spectrograph—allowed the team to analyze the spectral data and pinpoint the impacts of the quasar’s passage through the galaxy.
While this is the first direct observation of such an event, astronomers believe more powerful telescopes could soon reveal similar phenomena. Noterdaeme noted that ESO’s forthcoming Extremely Large Telescope “will certainly allow us to push forward a deeper study of this, and other systems, to better understand the evolution of quasars and their effect on host and nearby galaxies.
The paper “Quasar Radiation Transforms the Gas in a Merging Companion Galaxy” appeared on May 21, 2025, in Nature.
Ryan Whalen covers science and technology for The Debrief. He holds an MA in History and a Master of Library and Information Science with a certificate in Data Science. He can be contacted at ryan@thedebrief.org, and follow him on Twitter @mdntwvlf.