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'Cosmic joust': Astronomers observe pair of galaxies in deep-space battle

Astronomers have witnessed for the first time a violent cosmic collision in which one galaxy pierces another with intense radiation. Their results, in Nature, show that this radiation dampens the wounded galaxy's ability to form new stars.

This new study combined observations from both the European Southern Observatory's Very Large Telescope (ESO's VLT) and the Atacama Large Millimeter/submillimeter Array (ALMA), revealing all the gory details of this galactic battle.

In the distant depths of the universe, two galaxies are locked in a thrilling war. 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,'" says study co-lead Pasquier Noterdaeme, a researcher at the Institut d'Astrophysique de Paris, France, and the French-Chilean Laboratory for Astronomy in Chile, drawing a comparison to the medieval sport. 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.

"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," explains study co-lead Sergei Balashev, who is a researcher at the Ioffe Institute in St Petersburg, Russia.

The new observations indicate that radiation released by the quasar 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. Balashev explains, "These mergers are thought to bring huge amounts of gas to supermassive black holes residing in galaxy centers." 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.

This study was conducted using ALMA and the X-shooter instrument on ESO's VLT, both located in Chile's Atacama Desert.

ALMA's high resolution helped the astronomers clearly distinguish the two merging galaxies, which are so close together they looked like a single object in previous observations. With X-shooter, researchers analyzed the quasar's light as it passed through the regular galaxy. This allowed the team to study how this galaxy suffered from the quasar's radiation in this cosmic fight.

Observations with larger, more powerful telescopes could reveal more about collisions like this. As Noterdaeme says, a telescope like ESO's 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."