Black holes with different light signatures that were thought to be the same objects viewed from different angles are actually in different stages of their life cycles, according to a study by Dartmouth scientists. Research into black holes known as “active galactic nuclei,” or AGN, says it definitively shows the need to revise the widely used “unified AGN model,” which characterizes supermassive black holes as all having the same properties.”These objects have mystified scientists for more than half a century,” said TonimaTasnimAnanna, a postdoctoral researcher 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 obscured black holes differ significantly from those of AGNs, which are not so heavily obscured.”
Supermassive black holes are thought to reside at the center of almost all large galaxies, including the Milky Way. The objects gobble up galactic gas, dust, and stars and can be 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 in a phase of rapid growth.Beginning in the late 1980s, astronomers realized that light signatures coming from space from radio wavelengths to X-rays could be attributed to AGNs. The objects were thought to typically have a donut-shaped ring – or “torus” – 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 torus obscured the view.
From this, the unified theory of AGN became the prevailing understanding. The theory says that if a black hole is viewed through its torus, it should appear faint. If viewed from below or above the ring, it should appear bright. However, according to the current study, past research relied too much on data from less obscured objects and skewed the research results. The new study focuses on how quickly black holes feed on cosmic matter, or the rate of their accretion. The research found that the rate of accretion does not depend on the mass of the black hole, but varies significantly depending on how shrouded it is by the ring of gas and dust.
“This provides support for the idea that torus 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 grows.”The result shows that the amount of dust and gas surrounding an AGN is directly related to the amount it feeds, confirming that there are non-orientational differences between different AGN populations. When a black hole is accreting at high speed, the energy blows away the dust and gas. As a result, it is more likely to be uncovered and appear brighter. Conversely, less active AGN are surrounded by a denser torus and appear fainter.
“In the past, it was uncertain how the obscured AGN population differed from their more easily observable, non-obscured counterparts,” Ananna said. “This new research definitively shows a fundamental difference between the two populations that goes beyond point of view.”The study is based on 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 and detect obscured and unobscured AGNs.The research is the result of an international scientific collaboration the BAT AGN Spectroscopic Survey (BASS) which has been working for more than a decade to collect and analyze optical/infrared spectroscopy for AGN observed by Swift BAT.
“Never before have we had such a large X-ray sample obscured by a local AGN,” Ananna said. “This is a big win for high-energy X-ray telescopes.”The paper builds on previous research by the research team analyzing AGN. For the study, Ananna developed a computational technique to assess the effect of obscuring matter on the observed properties of black holes, and used the technique to analyze data collected by the wider research team.According to this paper, by knowing the mass of a black hole and its feeding rate, scientists can determine when most supermassive black holes experienced most of their growth, 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 touchstone for this research discipline.Future research could include focusing on wavelengths that will allow the team to search beyond the local universe. In the near future, the team would like to understand what triggers AGN to enter the high accretion regime and how long it takes for rapidly accreting AGNs to transition from heavily obscured to unobscured.
For more read: Tonima Tasnim Ananna, et. al, BASS. XXX. Distribution Functions of DR2 Eddington Ratios, Black Hole Masses, and X-Ray Luminosities. The Astrophysical Journal Supplement Series, 2022; 261 (1): 9 DOI: 10.3847/1538-4365/ac5b64
