A new count of the brightest celestial objects brings us closer to solving the mystery of why some of them have disappeared.
It’s the greatest disappearance phenomenon in the universe. Quasars —bright beacons powered by supermassive black holes lurking at the heart of distant galaxies—have been observed vanishing, sometimes in less than a year. Then, ordinary galaxies emerge in this dim glow. Although astronomers have long known that any quasar will eventually die, as its central black hole exhausts its raw material of gas and dust, these objects are so massive that this process would inevitably take tens of thousands of years. How, then, can this happen in less than a year?
One suggestion is that dust clouds passing in front of quasars may have temporarily obscured their view. Some theorists have argued that these strange objects could be something else entirely: when a supermassive black hole devours a star, it can cause a bright, brief flash. Or perhaps a passing sun acts as a celestial magnifying lens, amplifying the glare. But most of these ideas are unlikely, and instead, astronomers say the evidence points to the quasars themselves changing. Specifically, the real culprit lies in some alteration to their accretion disk, the ring of hot material that surrounds the black hole that is feeding on it. Now, this theory is supported by a new study in its preliminary version, recently accepted for publication in the Proceedings of the National Academy of Sciences of the United States of America . Astronomer Matthew Graham and his colleagues at the California Institute of Technology nearly tripled the number of quasars with a changing appearance, all of which showed distinct changes in their accretion disk. This is the largest sample of quasars ever collected, and it has given astronomers the opportunity to look deeper into the details behind these mysterious changes, which in turn may contribute to better explanations of the mechanisms by which black holes feed and how galaxies like our Milky Way have evolved over cosmic time.
Since the discovery of the first known quasar in 2014, astronomers have been diligently searching for more of these strange objects, some of which appear to brighten incredibly fast as a galaxy suddenly transforms into a luminous quasar. In this case, too, the time frame seemed impossible. Therefore, astronomers have been observing these objects at every available wavelength. More recently, this approach has included both visible light and infrared radiation—two wavelengths that allow researchers to explore different aspects of the physical system. In a quasar, visible light is often emitted from the accretion disk, while infrared radiation comes primarily from a larger, more distant, donut-shaped ring of dust surrounding the accretion disk. These two overlapping observations have revealed a surprising new detail: the observed changes in the visible light spectrum are offset by changes in the infrared radiation. Because the bright accretion disk emits light toward the dark ring, where it is absorbed and re-emitted as infrared waves, this simulation provides strong evidence of some kind of rapid change occurring within the accretion disk itself. “I’d bet a pretty penny—probably less than my mortgage or student loans—that changes in the net accretion rate are a plausible explanation,” says Nicholas Ross, co-author of the new study and an astronomer at the University of Edinburgh in Scotland.
In light of this model, Graham, Ross, and their colleagues carefully reviewed data from the Catalina Sky Survey, which includes measurements of photonic curves (measurements of light brightness as a function of time) over a decade from approximately half a billion sources in the sky. First, the team examined Catalina data to find star-like objects based on their spectra (the different colors of their emitted light). Second, they analyzed visible-light curves to identify star-like objects that had dimmed over time and compared them to infrared curves (using data from NASA’s Wide-field Infrared Survey Explorer) to see how each object dimmed in both types of light. Finally, the researchers studied the spectrum emitted by each object after it had dimmed again to confirm that it had transformed into a galaxy. They then looked for galaxies that had transformed into star-like objects.
In total, the team discovered 111 rapidly changing quasars, in addition to about 60 previously known ones. But this wasn’t simply adding data to an existing catalog. The new study represents a real-world, rigorous test of an emerging model of the physical conditions responsible for this enigmatic phenomenon. This achievement marks a significant milestone in the research process, says Eric Morganson, an astronomer at the University of Illinois at Urbana-Champaign, who was not involved in the study. All the strange new astronomical phenomena—even the quasars themselves, discovered in the mid-20th century—first entered the field through “sparkling” research that primarily pointed to their oddity. Astronomers begin with a few puzzling examples. But as they diligently and systematically investigate, they gain a deeper understanding, allowing them to develop better models of what they believe is actually happening. Finally, the research enters a new phase, where observations often serve to revise existing models rather than create entirely new ones. This study, Morganson says, is a watershed moment.
In the new sample, each quasar individually exhibited a change in the visible light spectrum, followed by a change in the infrared spectrum. The researchers believe this is sufficient evidence to conclude that the observed change is inherent to the quasar itself, and not the result of an external event or force. Therefore, the team argues that instead of describing these quasars as “changing-looking,” it would be more appropriate to describe them as “changing-state,” reflecting the true nature of their origins.
Regardless of the scientific terminology, the core of the mystery remains: How can an entire quasar suddenly dim? And how can a galaxy suddenly brighten? Researchers favor an explanation involving changes in the accretion disk’s temperature, caused by cold or hot gas fronts, shifts in magnetic fields, or a combination of both. Whatever the root cause, their study provides an example of what an interstellar accretion disk must look like for such changes to propagate so rapidly. Specifically, the team discovered that the disk must be extremely thick and viscous for such rapid changes to occur. “You can send a wave through molasses faster than through water,” says Kathleen E. Savic Ford, an astronomer at the American Museum of Natural History (AMNH) in New York City and Manhattan Community College (BMCC) at the City University of New York. “The same is true for a thicker, more bulging disk, somewhat like a doughnut or a CD,” Ford says. “You can think of it like a pipe. A thick pipe transmits information faster than a thin one.” Therefore, regardless of the nature of the change, it will likely require a swollen and sticky disc.
This finding contradicts the long-held scientific view that these disks should be thin. However, says Barry McKernan, co-author and an astronomer at the American Museum of Natural History in New York City and Manhattan Community College at the City University of New York, the result aligns with modern theories suggesting that accretion disks may be quite thick.
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