Mystery of the brightest objects in the universe finally solved

Quasars are the brightest objects in the universe, packing as much energy as a trillion stars into a volume the size of our solar system, and we now know how they form.
John Loeffler
Artist’s rendering of quasar P172+18
Artist’s rendering of quasar P172+18

ESO-M Kornmesser 

Quasars are the brightest, most powerful objects in the universe, but in the 60 years since they were first discovered, scientists couldn't explain how they formed, until now.

In a new study, published in the Monthly Notices of the Royal Astronomical Society, galaxy mergers are identified as the triggering event that produces these enormous galactic furnaces.

Quasars can pack as much energy as a trillion stars into a space the size of our solar system, making them the most powerful, and brightest, objects in our universe. But in the decades since they were discovered, there has been considerable debate about what could trigger such an explosion of energy.

Now, thanks to observations from researchers at the Universities of Sheffield and Hertfordshire, the evidence appears to fall firmly with galaxy mergers as the triggering event for quasar formation.

The researchers, using the Isaac Newton Telescope in La Palma, studied 48 galaxies with quasars and 100 galaxies without quasars and found that the galaxies with quasars were three times as likely to be directly interacting or actively colliding with other galaxies.

Most galaxies have a supermassive black hole at their center, which can normally be very bright if it is an active galactic nucleus, meaning that it is actively consuming vast amounts of gas. Before the gas gets swallowed up by the black hole though, it is accelerated to incredible speeds, and the interaction of the gas in a black hole's accretion disk emits powerful radiation that, ironically, makes active supermassive black holes very bright.

With quasars, however, this process is kicked into overdrive. Even though supermassive black holes might be consuming a great deal of gas, the overwhelming bulk of a galaxy's gas is contained much further away from the central black hole. The merger of galaxies ends up pushing a lot of this gas in towards the supermassive black hole at a galaxy's center, giving it far more gas to feed on than is typical.

“[Quasars are] an area that scientists around the world are keen to learn more about," the study's lead author, Dr. Jonny Pierce, Post-Doctoral Research Fellow at the University of Hertfordshire, said in a statement, "one of the main scientific motivations for NASA’s James Webb Space Telescope was to study the earliest galaxies in the Universe, and Webb is capable of detecting light from even the most distant quasars, emitted nearly 13 billion years ago. Quasars play a key role in our understanding of the history of the Universe, and possibly also the future of the Milky Way.”

Implications for the future of our galaxy

As Dr. Pierce indicates, this new study presents an interesting prospect for the future of our galaxy: we're likely to host a quasar in the future.

It's long been known that the Milky Way galaxy is on a collision course with the Andromeda galaxy, our nearest galactic neighbor, and if this new study is correct, then once that happens it's possible that Sagitarrius A*, the supermassive black hole at the heart of the Milky Way could become a quasar in the process.

"Quasars are one of the most extreme phenomena in the Universe, and what we see is likely to represent the future of our own Milky Way galaxy when it collides with the Andromeda galaxy in about five billion years," said Professor Clive Tadhunter, from the University of Sheffield’s Department of Physics and Astronomy, who co-authored this week's paper.

“It’s exciting to observe these events and finally understand why they occur – but thankfully Earth won’t be anywhere near one of these apocalyptic episodes for quite some time.”

The Milky Way and Andromeda aren't expected to collide for another 5 billion years or so, so the implications for us humans are fairly academic. But the consequences of the galaxy collisions are hardly harmless.

Since quasars are produced by the surge of gas from the outer parts of the galaxy during a merger, this ends up leaving far less gas in the galaxy for new star formation, which can stall a galaxy out for billions of years.

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