Astrophysicists at the University of Bath have developed a new method to locate extremely rare extragalactic objects. They hope their technique for finding “appearance-changing quasars” will take scientists one step closer to unraveling one of the universe’s greatest mysteries: the growth of supermassive black holes. Quasars are believed to be responsible for regulating the growth of supermassive black holes and their host galaxies.
A quasar is a dramatically luminous region in the center of a galaxy, fed by a supermassive black hole – the largest type of black hole, with a mass that exceeds that of our sun by millions or billions. There is a supermassive black hole in the center of the Milky Way.
Appearance-changing quasars quickly switch from a high-light state to a low-light state, and scientists have yet to understand why. When the brightness is reduced, a quasar is too dim to be seen in the context of the host galaxy, making it difficult for space scientists to find it or the supermassive black hole to which it is connected.
The new detection method will allow researchers to find quasars undergoing extreme changes in brightness, and thus create a more complete census of supermassive black holes. The next step will be to study the causes of brightness switches, to give scientists a better understanding of the growth of supermassive black holes. From there, clues are likely to emerge on the chain of events that give rise to the growth of galaxies, as the energy production of supermassive black holes can affect the fate of galaxies.
Astrophysicist Carolin Villforth, who participated in the research, said: “These supermassive quasars and black holes are extremely important to the evolution of galaxies – the more we learn about them, the more we understand how they influence the growth of galaxies. . “
WHAT EXACTLY ARE QUASARS?
Quasars are the brightest source of persistent light in the universe. Many galaxies, including our own, are believed to have one, and astrophysicists have identified over a million in total.
Quasars are formed when gaseous matter is attracted by gravitational forces to a supermassive black hole. As this gas approaches the black hole, it forms an “accretion disk” which orbits the black hole. Energy is released from the disk in the form of electromagnetic radiation, and it is this radiation that produces the brightness of the quasar.
The accretion disc is surrounded by a thick, dusty donut that masks much of the quasar’s emission. Because the dusty structure is very large, the level of obscuration should not change on a human scale, however a changing appearance quasar may appear to change from light to dark quickly (within a human year), which would be very surprising if that was true. Creating a more comprehensive list of changing-looking quasars would be a major step toward understanding the reasons for these apparent transitions.
Previous efforts to identify aspect-changing quasars have relied on variability over a wide range of wavelengths – a technique called photometric variability, which is known to miss lower-light quasars. The Bath researchers used spectroscopic data to assess changes in very small ranges of wavelengths, allowing them to detect aspect-changing quasars that had been missed by photometric research. Using this technique, they spotted four changing-looking quasars millions of light years from Earth. All four were too dark to be detected by photometric searches. Previous identification efforts had found only two of these quasars in the same area.
Former MPhys student in Bath, Bart Potts, who led the research, explained: “We took a previous dataset and applied our new method to see if we could identify any new changing quasars that others had. This gave us a larger set of changes – look at the quasars for further study, and validated that our methodology was more sensitive than the others, which was great. It shows that our methodology is more sensitive to a lower brightness. “
He added, “Ultimately, this discovery will bring something to the academic community that studies quasars. This will help other people to further their research on why this specific type of quasar goes through brightness switches. We help our community find important answers to big questions. “
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