Scientists have solved a major mystery about the most radiant and powerful objects in the universe, known as quasars, which are trillions of times brighter than the Sun and sit at the center of active galaxies, reports a new study.
The new research revealed that some quasars are triggered by collisions between galaxies, an epic process that hurls gas and dust into the supermassive black holes that exist at galactic centers. As hungry black holes gobble up these gassy meals, they unleash huge beams of energy that can be seen from across the observable universe. Some of these beams are pointed toward Earth, creating an optical illusion that makes them appear to break the laws of physics.
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While previous studies have hinted that galactic mergers can power quasars, a host of new observations now expose unprecedented evidence for this origin. The findings may even offer a glimpse of the future of our Milky Way galaxy, which could turn into a quasar when it collides with the nearby Andromeda galaxy in 4.5 billion years.
Quasars are a subclass of active galactic nuclei (AGN), which are compact and luminous galactic cores that are powered by supermassive black holes. As the most radiant and pyrotechnic type of AGN, quasars are known for spewing out radiation all across the light spectrum, distinguishing them as the brightest objects in the universe.
The exceptional brilliance of quasars can make them easy to spot, yet the exact mechanisms that ignite these powerhouses remain unclear. Now, scientists led by Jonathon Pierce, a postdoctoral research fellow at the University of Hertfordshire, have shed new light on this puzzle by observing 48 quasars that exist within about two billions light years of Earth.
The team discovered “clear evidence that galaxy interactions are the dominant triggering mechanism for quasar activity in the local universe,” according to a study published on Wednesday in the Monthly Notices of the Royal Astronomical Society.
“Galaxy collisions present a particularly attractive means for igniting quasar activity, since they are capable of driving large amounts of fuelling gas to the central galaxy locations where the activity manifests itself,” Pierce told Motherboard in an email.
“While previous searches for signatures of galaxy interactions in quasar hosting galaxies have given mixed results, we believe that this is because the images used in many of the studies were not sensitive enough to detect them,” he added. “In an attempt to tackle this issue, we decided to image a large sample of quasars with the appropriate depth to identify these signatures, if they were present.”
To that end, Pierce and his colleagues gathered a comprehensive dataset of wide-field observations from the Isaac Newton Telescope (INT) in La Palma. The researchers imaged 48 “type 2” quasars, which are quasars that are obscured by large amounts of gas and dust. “Type 1” quasars, in contrast, are not blocked by any material, giving them an extremely bright star-like appearance.
“We chose to use type 2 quasars for this study because of their ‘hidden’ nature,” explained Pierce. “The very bright, star-like appearance of type 1 quasars can cause complications with the images that make it more difficult to identify the presence of galaxy interaction signatures, due to saturation and other issues. Type 2 quasars, on the other hand, do not suffer from these issues, since the direct light from the quasar is obscured.”
In addition to observing these murky quasars, the team examined more than 100 regular galaxies without AGN that were used as a control sample. Because the control galaxies were observed simultaneously with the quasars using the INT’s Wide Field Camera, the researchers were able to ensure that the image quality and depth were the same, on average, for both groups.
“We chose the Isaac Newton Telescope because of its ability to image with the sensitivity required to detect the galaxy interaction signatures we were looking for, and, to our knowledge, our sample of 48 type 2 quasars is the largest of its kind to be imaged at this depth,” Pierce said.
“Many of the previous studies in this area did not use a control sample or did not image with sufficient depth, which opens up the possibility that the results could have been biased by the chosen imaging setup or the properties of the particular samples of quasars used,” he added.
The researchers searched for signs of galactic mergers in this dataset, especially tidal effects that warp galactic structures. They discovered that the active galaxies hosting quasars were three times more likely to show signs of interactions or full-on collisions with other galaxies, suggesting that these galactic mergers are the most common triggers for type 2 quasars in the local universe.
“While there were hints from the results of other studies that had imaged with suitable sensitivity, we still did not know what we would find,” Pierce said. “Our identifications of interaction signatures were done randomly and blindly (without knowing if we were looking at a quasar or non-quasar galaxy), so we didn’t have an idea of the results we would get until later in the study.”
“To be good scientists, I guess we wouldn’t say that we are happy with the results, but we hope that we have helped to bring some clarity to the picture of what could trigger these powerful objects!” he noted.
Indeed, the discovery confirms that the unions of galaxies create turbulent environments that feed supermassive black holes and spark explosive type 2 quasars. Understanding these changes is important in part because the transition from an inactive galaxy to a quasar can have serious repercussions for galactic systems. For instance, quasars can dramatically reduce star formation within galaxies by essentially blowing all the star-making gas out into the intergalactic wilds.
While the new study yields insights about these cataclysmic events, Pierce and his colleagues noted that galactic mergers are not the only source of quasars. Even some of the AGN observed in this study didn’t match the galactic merger model, suggesting that other mechanisms may be at play in these extreme systems. In other words, quasars still have many secrets left to divulge, leaving astronomers with plenty of work left to do.
“Quasars can be identified via several different methods (e.g. like the type 1/type 2 selection methods mentioned above), some of which use light from different parts of the electromagnetic spectrum (for instance via bright emission at X-ray or infrared wavelengths),” Pierce concluded. “These different methods could select quasars with different properties, and so it would be good to confirm that galaxy collisions are important for triggering all quasars, regardless of how they were selected.”
