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There's Something More Than Dust Hiding the Black Hole Hearts of Galaxies

A nearly 50 year old model gets a bit more cloudy.
Image: NASA

Since the 1970s, astronomers have had a theory about black holes: every single one of them is surrounded by a doughnut-shaped cloud of dust. But a recent survey of more than 170,000 supermassive black holes conducted with help from NASA's Wide-field Infrared Survey Explorer (WISE) has poked new holes in the long-accepted doughnut theory.

The unified theory of supermassive black holes is nearly 50 years old. It explains why these cosmic objects, which lie at the centre of every galaxy and are all similar in nature, can look vastly different, sometimes shrouded in dust and sometimes clear. The key is the doughnut of dust that surrounds all of them: the dust doughnut is angled as seen from the Earth, meaning sometimes it’s thick and sometimes it’s basically invisible.

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But the latest data from WISE, which scans and images the sky in infrared light to reveal unseen features, flies in the face of this long-held theory. Scientists have found that in some cases, something other than a doughnut structure may obscure certain cloudy-looking black holes.

What that something is, however, is still a mystery. ”Our finding revealed a new feature about active black holes we never knew before, yet the details remain a mystery," said Lin Yan of NASA's Infrared Processing and Analysis Center at the California Institute of Technology in Pasadena. "We hope our work will inspire future studies to better understand these fascinating objects."

Yan and his team's study is focused on feeding or active black holes, those that are currently taking in all their surrounding gas material to fuel their growth. They looked at more than 170,000 active black holes as captured in WISE data then measured how these objects are clumped together.

If the unified model were true—if galaxies looked different solely because of the angle at which we see the gas cloud—then all galaxies should group together the same way. But the WISE data found galaxies are a little more selective in how they clump. It turns out that galaxies with obscured black holes are more clumped together than those with exposed black holes at their cores.

"The main purpose of unification was to put a zoo of different kinds of active nuclei under a single umbrella," said Emilio Donoso, a post-doctoral researcher who worked with Yan and is currently with the Instituto de Ciencias Astronómicas, de la Tierra y del Espacio in Argentina. “Now, that has become increasingly complex to do as we dig deeper into the WISE data.” If the new findings are confirmed, it will mean adjusting the unified model to explain why some black holes are hidden.

It’s possible the answer lies with dark matter, the invisible substance that accounts for a large majority of all matter in the universe. Every galaxy sits inside a dark matter halo, and the bigger the halo the more gravity. With more gravity, the galaxies will be pulled closer together.

"The unified theory was proposed to explain the complexity of what astronomers were seeing," said  Daniel Stern, a scientists with NASA’s Jet Propulsion Laboratory 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.'"

There’s still a lot of work to be done to confirm this latest discovery and more WISE data to sort through. One thing’s for sure—our universe is a stranger place than we know, or we thought we knew.