Doughnut Theory May Not Adequately Explain Hidden Black Holes
A decades-old theory used by astronomers to explain the varying appearances of supermassive black holes is being called into question in the wake of new data obtained using NASA's Wide-field Infrared Survey Explorer (WISE), officials from the US space agency announced on Thursday.
The unified theory of active, supermassive black holes was devised in the late 1970s to explain why black holes that are similar in many ways can have radically different appearances. Some of them appear as though they're shrouded in dust, while others are in the open and easily visible.
The theory posits that every black hole is surrounded by what is known as a torus - a dusty, doughnut-shaped structure that alters the appearance of the black hole based on its orientation. For instance, a black hole is visible if the torus is observed from above or below, but is hidden if it is positioned so that we see it edge-on.
However, the results of a new survey of over 170,000 supermassive black holes using WISE do not corroborate the so-called doughnut theory. The investigators behind the research uncovered evidence that, in some cases, something other than the torus could determine whether or not a black hole is hidden or visible. They are not yet certain what that "something" might be, but their findings nonetheless cast doubts on the unified model.
"Our finding revealed a new feature about active black holes we never knew before, yet the details remain a mystery," Lin Yan of the NASA Infrared Processing and Analysis Center (IPAC), second author of a paper that has been accepted for publication by the Astrophysical Journal, explained. "We hope our work will inspire future studies to better understand these fascinating objects."
Yan, lead author Emilio Donoso from the Instituto de Ciencias Astronómicas, de la Tierra y del Espacio in Argentina, and their colleagues used computers to select more than 170,000 active supermassive black holes from the WISE data. Then they measured the clustering of galaxies that contained both hidden and exposed black holes.
As NASA explained, all galaxies have a massive black hole at their core, and the active ones feed on surrounding gas material that helps spur on their growth. If the unified model was true and the hidden black holes were obscured by doughnuts in the edge-on configuration, they and the exposed ones should cluster in the same way.
"According to theory, since the doughnut structures would take on random orientations, the black holes should also be distributed randomly," the space agency said. "It is like tossing a bunch of glazed doughnuts in the air - roughly the same percentage of doughnuts always will be positioned in the edge-on and face-on positions, regardless of whether they are tightly clumped or spread far apart."
However, the analysis of the WISE data revealed that the galaxies that have hidden black holes are more clumped together than the ones that contain exposed black holes. The findings still need to be confirmed, but if they are, scientists will have to tweak the unified model with new explanations for why some black holes are hidden.
One potential explanation for the results involves dark matter, the invisible substance found throughout the universe. Every galaxy is surrounded by a dark matter halo, NASA explained, and since the obscured black holes are more clustered than the others, their galaxies possess larger dark matter halos. While the halos themselves are not the reason that the black holes appear hidden, they could provide insight into the actual cause.
"The unified theory was proposed to explain the complexity of what astronomers were seeing," said co-author Daniel Stern of NASA's Jet Propulsion Laboratory (JPL) in Pasadena, California. "It seems that simple model may have been too simple. As Einstein said, models should be made 'as simple as possible, but not simpler'."