Ancient galaxy collision reveals a quasar shutting down star birth

A pair of far-flung galaxies has grabbed the spotlight by engaging in a high-speed collision that stretches our understanding of cosmic interactions. One galaxy is blasting the other with intense radiation, and astronomers say this barrage is limiting new star formation in the battered galaxy.

The study was led by Pasquier Noterdaeme from the Paris Institute of Astrophysics and the French-Chilean Laboratory for Astronomy in Chile and Sergei Balashev from the Ioffe Institute in St Petersburg, Russia.

The researchers noted that these two galaxies are racing toward each other at about 310 miles per second. Their study describes how this collision plays out when one galaxy packs a powerful quasar weapon.

Galaxy collision and a quasar blast

The galaxies lie more than 11 billion years in the past. Our current observations reveal them when the universe was only about 18% of its present age.

“We hence call this system the ‘cosmic joust,’” said Noterdaeme. One galaxy harbors a supermassive black hole that unleashes extreme radiation and transforms ordinary gas clouds in its unwary companion. 

A quasar forms when enormous amounts of matter spiral into a supermassive black hole, shining with brightness that can outmatch entire galaxies. Such intensity can wreak havoc on star-forming gas if it blasts through a neighboring galaxy.

“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 Balashev.

Observations reveal that only the smallest, densest pockets of gas remain intact, which appear incapable of supporting much star formation.

Evolution of galaxies and quasars

The intense radiation from the quasar didn’t just scatter the gas – it compressed what was left into tiny, dense clumps. These molecular clouds are far smaller than typical star-forming regions, some measuring less than 0.02 light-years across.

Astronomers estimate that these compact clouds are at least 100,000 times denser than the surrounding space. But even with their density, they’re too small to kickstart new stars.

This points to negative feedback, a process where energy from black holes actively shuts down star formation in surrounding areas.

Even though this galactic clash happened over 11 billion years ago, it’s giving scientists a rare window into how galaxies and quasars evolved in the early universe. Most of what we know about these processes comes from models or indirect observations.

Seeing a real-time example, frozen in light that just reached us, helps confirm long-standing theories about how radiation from black holes can shut down star formation in nearby galaxies.

Telescopes reveal two galaxies

By combining data from the Atacama Large Millimeter/submillimeter Array and the Very Large Telescope, researchers were able to study the quasar’s light as it carved through the second galaxy. They found that what once looked like a single fuzzy object in older images turned out to be two galaxies in a fierce collision.

The team identified disruptions in the attacked galaxy’s gas clouds, linking them to the quasar’s intense radiation. Astronomers also discovered that the chaos supplies fresh gas to the ravenous black hole, prolonging its brutal assault.

The data came from two observatories located in the Atacama Desert, one of the driest places on Earth. These high-altitude sites offer some of the clearest views of the night sky, with minimal atmospheric interference.

ALMA’s ability to detect fine detail in millimeter wavelengths, combined with the VLT’s precise light analysis, made it possible to separate the two galaxies and trace the radiation’s path through the victim galaxy.

The aftermath of galaxy collisions

“This 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,” said Noterdaeme.

The researchers predict that future instruments like the Extremely Large Telescope could investigate the aftermath of such collisions in closer detail. 

When galaxies collide, they don’t just create chaos, they also feed the supermassive black holes at their centers. These mergers funnel gas and dust inward, giving black holes the fuel they need to flare up as quasars.

This process explains why quasars were more common in the early universe. Galaxy interactions were far more frequent, and each one offered another chance for a black hole to ignite and reshape its surroundings.

The study is published in the journal Nature.

Image Credit: ESO / M. Kornmesser.

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