Space study offers clearest understanding yet of supermassive black hole life cycle

Black holes with different light signatures, which were assumed to be the same objects when viewed from different angles, are actually at different stages of the life cycle, according to a study conducted by Dartmouth researchers.

Research on black holes, known as “active galactic nuclei” or AGNs, says it definitely shows the need to revise the widely held “unified model of AGN,” which characterizes supermassive black holes as all having the same properties.

The study, published in The Astrophysical Journalprovides answers to a troubling space mystery and should allow researchers to create more accurate models of the evolution of the Universe and the evolution of black holes.

“These objects have puzzled researchers for over half a century,” said Tonima Tasnim Ananna, a postdoctoral researcher at Dartmouth and lead author of the paper. “Over time we have made many assumptions about the physics of these objects. Now we know that the properties of eclipsed black holes differ significantly from the properties of less eclipsed AGNs.”

Supermassive black holes are believed to be at the center of almost all large galaxies, including the Milky Way. The objects engulf 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 cores, a type of supermassive black hole that is “accreting,” or in a rapid growth state.

Beginning in the late 1980s, astronomers realized that light signatures from space, ranging from radio wavelengths to X-rays, could be assigned 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 color associated with the objects was thought to be the result of the angle from which they were observed and how much of the torus was obscuring the view.

From this, the unified theory of AGNs became the prevailing understanding. The theory states that when viewed through its torus, a black hole should appear faint. When viewed from below or above the ring, it should appear bright. However, according to the current study, previous research relied too heavily on data from less obscured objects and biased research results.

The new study focuses on how fast black holes feed on space matter, or their accretion rates. The research found that the rate of accretion does not depend on a black hole’s mass, it varies significantly depending on how occluded it is by the ring of gas and dust.

“This supports the idea that the 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 the structure and its growth.”

The result shows that the amount of dust and gas surrounding an AGN is directly related to how much it feeds, confirming that there are differences between different AGN populations beyond orientation. When a black hole accretes at high speed, the energy blows dust and gas away. This makes it more likely that it will not be obscured and appear lighter. Conversely, a less active AGN is surrounded by a denser torus and appears fainter.

“Historically, it has been uncertain how the occult AGN population differs from their more easily observable non-occult counterparts,” Ananna said. “This new research definitely shows a fundamental difference between the two populations that goes beyond perspective.”

The study stems from a decade-long analysis of nearby AGNs discovered by Swift-BAT, NASA’s high-energy X-ray telescope. The telescope allows researchers to scan the local Universe to discover occluded and non-occluded AGNs.

The research is the result of an international scientific collaboration – the BAT AGN Spectroscopic Survey (BASS) – which has worked for over a decade to collect and analyze optical/infrared spectroscopy for AGN observed by Swift BAT.

“We’ve never had such a large sample of X-ray-detected covert local AGN,” Ananna said. “This is a great win for high-energy X-ray telescopes.”

The paper builds on previous research by the research team analyzing AGNs. For the study, Ananna developed a computational technique to assess the effect of matter darkening on observed black hole properties and analyzed data collected by the broader research team using the technique.

According to the paper, by knowing the mass of a black hole and how fast it feeds, researchers can determine when most supermassive black holes have completed most of their growth, providing valuable information about the evolution of black holes and of the universe deliver.

“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 that question and I expect it will become a touchstone reference for this research discipline.”

Future research could focus on wavelengths that allow the team to search beyond the local Universe. In the short term, the team wants to understand what triggers AGNs to switch to high accretion mode and how long it takes for rapidly accumulating AGNs to transition from highly occluded to unoccluded.