Two galaxies 'joust' before cosmic mega-merger

Astronomers have observed two distant galaxies – each containing roughly as many stars as the Milky Way – hurtling towards each other ahead of an inevitable merger, in a scene resembling two knights charging in a joust. This cosmic event took place when the universe was about a fifth of its current age.

The galaxies, observed using two Chile-based telescopes, are seen as they existed approximately 11.4 billion years ago – around 2.4 billion years after the Big Bang, which marked the beginning of the universe.

At the heart of one of the galaxies lies a quasar – a highly luminous object powered by gas and other material falling into a supermassive black hole. The intense radiation emitted across the electromagnetic spectrum by the quasar is observed disrupting clouds of gas and dust, known as molecular clouds, in the companion galaxy.

Molecular clouds are the birthplaces of stars. However, the quasar’s radiation transformed the clouds in the affected galaxy into “only tiny dense cloudlets that are too small to form stars,” said astrophysicist Sergei Balashev of the Ioffe Institute in Saint Petersburg, Russia, co-lead author of the study published on Wednesday in the journal Nature.

This is the first time such a phenomenon has been observed, Balashev said.

Stars form through the gradual gravitational contraction of molecular clouds, where small central regions heat up and ignite to become new stars. But in the galaxy affected by the quasar, fewer of these stellar nurseries could develop, leading to a suppressed star formation rate.

The interaction between the two galaxies reminded researchers of a medieval joust.

“Much like jousting knights charging toward one another, these galaxies are rapidly approaching. One of them – the quasar host – emits a powerful beam of radiation that pierces the companion galaxy, like a lance. This radiation ‘wounds’ its ‘opponent’ as it disrupts the gas,” said astronomer and co-lead author Pasquier Noterdaeme of the Paris Institute of Astrophysics in France.

Supermassive black holes are found at the centre of many galaxies, including the Milky Way. The black hole powering the quasar in this study is estimated to be about 200 million times the mass of our sun.

The immense gravitational pull of the supermassive black hole draws in gas and other material. As this matter spirals inward at high speed, it heats up due to friction, forming a disk that emits extremely powerful radiation in two opposing directions – known as biconical beams.

It was ultraviolet light from one of these beams that disrupted the gas in the companion galaxy.

This supermassive black hole is vastly more massive than the one at the centre of the Milky Way – known as Sagittarius A*, or Sgr A* – which has a mass of approximately 4 million suns and is located about 26,000 light years from Earth. A light year is the distance light travels in one year: about 5.9 trillion miles (9.5 trillion kilometres).

The researchers used the Atacama Large Millimeter/submillimeter Array (ALMA) to characterise the two galaxies and the European Southern Observatory’s Very Large Telescope (VLT) to examine the quasar and the gas in the companion galaxy.

The alignment of the galaxies from Earth’s vantage point allowed researchers to observe the quasar’s radiation passing directly through the companion galaxy.

Most galaxy mergers observed by astronomers have taken place later in the universe’s history.

“Galaxies are typically found in groups, and gravitational interactions naturally lead to mergers over cosmic time,” Noterdaeme said. “In line with current understanding, these two galaxies will eventually coalesce into a single larger galaxy. The quasar will fade as it exhausts the available fuel.”