Intense radiation from a quasar in one of these galaxies alters the gas properties in the other galaxy and dampens its ability to form new stars, according to an analysis of data from ESO’s Very Large Telescope (VLT) and the Atacama Large Millimeter/submillimeter Array (ALMA).
This artist’s impression shows a galactic merger in which the galaxy on the right hosts a quasar at its core; this quasar is powered by a supermassive black hole swallowing up material around it and emitting a powerful cone of radiation, piercing the other galaxy like a lance; as this radiation interacts with the galaxy on the left, it disrupts the clouds of gas and dust within, leaving behind only the smallest and densest regions; these regions are likely rendered incapable of star formation after the process. Image credit: ESO / M. Kornmesser.
“In the distant depths of the Universe, two galaxies are locked in a thrilling war,” said Dr. Pasquier Noterdaeme, an astronomer at the Institut d’Astrophysique de Paris and the French-Chilean Laboratory for Astronomy.
“Over and over, they charge towards each other at speeds of 500 km/s on a violent collision course, only to land a glancing blow before retreating and winding up for another round.”
“We hence call this system the ‘cosmic joust.’ But these galactic knights aren’t exactly chivalrous, and one has a very unfair advantage: it uses a quasar to pierce its opponent with a spear of radiation.”
Quasars are the bright cores of some distant galaxies that are powered by supermassive black holes, releasing huge amounts of radiation.
Both quasars and galaxy mergers used to be far more common, appearing more frequently in the Universe’s first few billion years, so to observe them astronomers peer into the distant past with powerful telescopes.
The light from this ‘cosmic joust’ has taken over 11 billion years to reach us, so we see it as it was when the Universe was only 18% of its current age.
This ALMA image shows the molecular gas content of two galaxies involved in the cosmic collision. Image credit: ALMA / ESO / NAOJ / NRAO / Balashev et al.
“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,” said Dr. Sergei Balashev, an astronomer at the Ioffe Institute.
The new VLT/ALMA observations indicate that radiation released by the quasar, known as J012555.11-012925.00, disrupts the clouds of gas and dust in the regular galaxy, leaving only the smallest, densest regions behind.
These regions are likely too small to be capable of star formation, leaving the wounded galaxy with fewer stellar nurseries in a dramatic transformation.
But this galactic victim isn’t all that is being transformed.
“These mergers are thought to bring huge amounts of gas to supermassive black holes residing in galaxy centres,” Dr. Balashev said.
“In the cosmic joust, new reserves of fuel are brought within reach of the black hole powering the quasar.”
“As the black hole feeds, the quasar can continue its damaging attack.”
A paper on the findings was published today in the journal Nature.
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S. Balashev et al. Quasar radiation transforms the gas in a merging companion galaxy. Nature, published online May 21, 2025; doi: 10.1038/s41586-025-08966-4
