Research into black holes known as “active galactic nuclei” or AGNs, he says, definitively shows the need to revise the widely used “unified AGN model” that characterizes supermassive black holes as all having the same properties. The study, published in The Astrophysical Journal, provides answers to a vexing space mystery and will allow researchers to create more accurate models of the evolution of the universe and how black holes grow. “These objects have puzzled researchers for over half a century,” said Tonima Tasnim Ananna, a postdoctoral research associate at Dartmouth and lead author of the paper. “Over time, we have made many assumptions about the physics of these objects. We now know that the properties of shadowed black holes are significantly different from the properties of AGNs that are not so obscured.” Supermassive black holes are believed to be at the center of almost all large galaxies, including the Milky Way. The objects gobble up galactic gas, dust and stars and can become heavier than small galaxies. For decades, researchers have been interested in the light signatures of active galactic nuclei, a type of supermassive black hole that is “accreting,” or rapidly growing. Beginning in the late 1980s, astronomers realized that space-derived light signatures ranging from radio wavelengths to X-rays could be attributed to AGNs. The objects were thought to typically have a donut-shaped — or “torus” — ring of gas and dust around them. The different brightness and colors associated with the objects were thought to be the result of the angle from which they were viewed and how much of the finger occluded the view. From this, the unified theory of AGNs became the prevailing understanding. Theory dictates that if a black hole is observed through its core, it should appear dim. If you see it from below or above the ring, it should look bright. According to the current study, however, previous research relied heavily on data from less obscure objects and skewed research results. The new study focuses on how quickly black holes are fed by space matter, or their accretion rates. The research found that the accretion rate does not depend on the mass of a black hole, but varies significantly depending on how much it is covered by the ring of gas and dust. “This supports the idea that the finger structures around black holes are not all the same,” said Ryan Hickox, professor of physics and astronomy and co-author of the study. “There is a relationship between structure and how it develops.” The result shows that the amount of dust and gas surrounding an AGN is directly related to how much it is fed, confirming that there are differences beyond orientation between different AGN populations. When a black hole is accreting at a high rate, the energy expels dust and gas. As a result, it is more likely to be unbounded and appear brighter. Conversely, a less active AGN is surrounded by a denser ring and appears fainter. “In the past, it was uncertain how the obscured AGN population differed from their more easily observable, unobscured counterparts,” Ananna said. “This new research conclusively shows a fundamental difference between the two populations that goes beyond the viewing angle.” The study comes from a ten-year analysis of nearby AGNs detected by Swift-BAT, NASA’s high-energy X-ray telescope. The telescope allows researchers to scan the local universe to detect dark and non-AGNs. The research is the result of an international scientific collaboration — the BAT AGN Spectroscopic Survey (BASS) — that has been working for more than a decade to collect and analyze optical/infrared spectroscopy for AGN observed by Swift BAT. “We have never before had such a large X-ray sample that has detected a dark local AGN,” Ananna said. “This is a big win for high-energy X-ray telescopes.” The work builds on previous research by the research team analyzing AGNs. For the study, Ananna developed a computational technique to assess the effect of obscuring matter on the observed properties of black holes and analyzed data collected by the larger research team using this technique. According to the paper, by knowing the mass of a black hole and how fast it is feeding, researchers can determine when most supermassive black holes underwent most of their growth, thus providing valuable information about the evolution of black holes and the universe. “One of the biggest questions in our field is where supermassive black holes come from,” Hickox said. “This research provides a critical piece that can help us answer this question, and I expect it to become a beacon for this research discipline.” Future research could include focusing on wavelengths that allow the team to look beyond the local universe. In the near future, the team would like to understand what causes AGNs to go into high-accretion mode and how long it takes for rapidly accreting AGNs to go from intensely dark to obscured. Researchers contributing to the study include Benny Trakhtenbrot, Tel Aviv University; Claudia Megan Urry, Yale University; and Mike Koss of Eureka Scientific.
