Astronomers just solved a 60-year mystery regarding the brightest objects in the universe

Until now, no one knew for sure what quasars — among the most massive explosions in the universe — really were

By Troy Farah

Science & Health Editor

Published April 30, 2023 1:59PM (EDT)

This artist's concept shows the brilliant glare of two quasars residing in the cores of two galaxies that are in the chaotic process of merging. (NASA, ESA, Joseph Olmsted (STScI))
This artist's concept shows the brilliant glare of two quasars residing in the cores of two galaxies that are in the chaotic process of merging. (NASA, ESA, Joseph Olmsted (STScI))

One of the most mysterious objects in the universe just got a little less mysterious.

For sixty years since they were first detected, astronomers have puzzled over what quasars were. What we knew was this: They are among the most bright and powerful objects in the entire universe. They are extremely distant from Earth, yet can glimmer as bright as a trillion stars while clustering into a space as small as our solar system — that's an extremely concentrated level of energy.

But until this week, astronomers weren't entirely sure what causes these extreme explosions in the first place.

Quasars are powered by supermassive black holes, and sometimes belch out waves of matter that can snuff out baby stars.

The term quasar is a concatenation of quasi-stellar radio source — so called because when they were first identified, astronomers like Hong-Yee Chiu, who coined the term, were completely baffled by these strange interstellar objects. Their intense gravity can create illusions of physics known as gravitational lensing, in which light is warped just like cosmic magnifying glasses. As recently as several decades ago, it was hard to even know which direction this lensing was coming from. 

As we've narrowed it down, we've learned that quasars sit at the center of galaxies, which are giant swirling discs of stars, gas, dust and dark matter strung together by gravity. That may seem self-evident, but galaxies have to spin around something. Most galaxies, ours included, have a central supermassive black hole upon which everything pivots — and so-named because these dead stars can be between one hundred thousand and ten billion times more massive than our Sun. The Milky Way spirals around a supermassive black hole with the lovely name Sagittarius A*. Since humans have had the telescope technology to detect black hole mergers — which admittedly hasn't been very long — we haven't yet detected the merger of two supermassive black holes. When we do, the explosion will be incalculable.

But some galaxies have much different objects at their center called an active galactic nucleus (AGN). These can be quite volatile, such as blazars, which are AGN that shoot out jets of ionized matter traveling at nearly the speed of light. But quasars are an AGN that are even more intense. They're powered by supermassive black holes, sometimes belching waves of matter that can snuff out baby stars. As such, they've become an essential part of our understanding of the early universe and galaxy evolution.

Unfortunately, despite their importance and awe-inspiring destructive power, quasars are difficult to study due to their extreme distance and brightness. They also don't have lifespans that are very long relative to when their triggering events occur, and their brightness can vary over time, further complicating observations and muddying data. All of this has made their origins unclear.

A new study in the journal Monthly Notices of the Royal Astronomical Society sheds light on the question of quasars, essentially solving one of the central mysteries of how quasars form. The answer could lie in galactic collisions.

As the researchers explain, colliding galaxies appear to create the conditions to birth a quasar. Indeed, these violent interweavings can cause enough gas to flow towards the core supermassive black holes, initiating quasar activity even before the two galaxies fully merge.

This theory has been posited before, but there has never been as solid direct evidence before now. The researchers, led by Jonathon Pierce, a postdoctoral research fellow at the University of Hertfordshire, observed nearly 50 galaxies that are host to quasars and compared them to more than 100 galaxies that are quasar-free. Similar comparisons have been made many times before, but this marks the first time so many quasars have been imaged with such sensitivity. They used deep imaging observations from the Isaac Newton Telescope in La Palma, one of Spain's Canary Islands, and concluded that galaxies hosting quasars are around three times more likely to be colliding or interacting with other galaxies.

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In the study, the authors said that their images of quasars "provide strong evidence that galaxy interactions are the dominant triggering mechanism for quasars in the local universe." This, they said, was consistent with quasars of different "brightnesses," meaning with varying strength to their radio emissions.

"Quasars play a key role in our understanding of the history of the Universe, and possibly also the future of the Milky Way."

All quasars are extremely far away, which means that they are also in the distant past, because of the way that distance correlates with earlier points of time as we gaze at far-away objects. However, there may be a quasar nearby in a few billion years, if you can wait that long. As the Milky Way collides with the Andromeda galaxy in a protracted process that will occur in roughly five billion years, the aftermath is likely to produce a quasar.

"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," Professor Clive Tadhunter, from the University of Sheffield's Department of Physics and Astronomy and one of the paper's co-authors, said in a statement. "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."

It's taken decades of work to get where we are now with our understanding of quasars, which is fundamental to shaping our theories on how the universe formed and where it's heading. As Pierce, the study's lead author explained in the same 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."

By Troy Farah

Troy Farah is Salon's science and health editor specializing in drug policy and pandemics.

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