Hubble Sees Dark Shadows That Could Be Cast by a Supermassive Black Hole

We use the term ‘supermassive black hole’ with a kind of casual familiarity. But stop and think about what they really are: Monstrous, beguiling singularities where the understood laws of physics and cosmology are brought to their knees. A region where gravity is so powerful that it warps everything around it, drawing material in—even light itself—and sometimes spitting out jets of energy at near-light-speed.

It was only recently that we got our first image of one of these monstrosities. Now, the Hubble has captured an image of a supermassive black hole (SMBH), or what might be part of its shadow, anyway.

An SMBH is defined as a black hole with millions to billions of times the mass of our Sun. They’re much more enormous than a stellar black hole. All black holes have their own anatomy, with an accretion disk of gas and dust, an event horizon, and the singularity at the center of it all.

This artist’s impression depicts a rapidly spinning supermassive black hole surrounded by an accretion disc. This thin disc of rotating material consists of the leftovers of a Sun-like star which was ripped apart by the tidal forces of the black hole. Image Credit: ESO

Recently, Hubble spotted what seems to be the shadow of an SMBH’s dust ring cast into space. The black hole is in IC 5063, a relatively nearby Seyfert galaxy only 156 million light-years from Earth. Scientists are no stranger to IC 5063; it’s been the subject of study for a while.

When a black hole draws material to itself, the material accumulates into an accretion disk. This material becomes superheated, and it generates an enormous amount of light. Scientists working with Hubble think that all that light is striking this SMBH’s inner dust ring—called a torus—which is buried deep inside the core. That’s creating the shadow of the dust ring at IC 5063.

Some of the light passes through gaps in the ring, producing an effect similar to the Sun setting in a partly cloudy sky here on Earth. That light passes through openings, creating visible beams called crepuscular rays. Those visible beams provide some clues to the structure of the dust ring itself.

Sunbeams in Nevada during a sunset. Image Credit: By ExorcisioTe – Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=71101650

The dust ring scenario is only one possible explanation. There’s no way to see inside to confirm it. But if it is correct, it means the inner dust ring has a clumpy nature.

A team of astronomers led by Peter Maksym of the Center for Astrophysics | Harvard & Smithsonian (CfA) have been studying the image. They’ve traced the light rays back to their source, which seems to be the SMBH at the galaxy’s center. The team have developed other scenarios to explain what they’re seeing, but according to them the black hole dust ring shadow explanation is the strongest.

They presented their research in a new paper titled “Crepuscular Rays from the Highly Inclined Active Galactic Nucleus in IC 5063.” Maksym is the lead author, and the paper is published in The Astrophysical Journal Letters.

This Hubble Space Telescope image of the heart of nearby active galaxy IC 5063 reveals a mixture of bright rays and dark shadows coming from the blazing core, home of a supermassive black hole. Astronomers suggest that a ring of dusty material surrounding the black hole may be casting its shadow into space. The observations were taken on March 7 and Nov. 25, 2019 by Hubble’s Wide Field Camera 3 and Advanced Camera for Surveys.
CREDITS: NASA, ESA, STScI and W.P. Maksym (CfA)

“I’m most excited by the shadow of the torus idea because it’s a really cool effect that I don’t think we’ve seen before in images, although it has been hypothesized,” Maksym said in a press release. “Scientifically, it’s showing us something that is hard—usually impossible—to see directly. We know this phenomenon should happen, but in this case, we can see the effects throughout the galaxy. Knowing more about the geometry of the torus will have implications for anybody trying to understand the behavior of supermassive black holes and their environments. As a galaxy evolves, it is shaped by its central black hole.”

The angle of the cones of light relative to the galaxy’s orientation are a big clue to the source of the rays and shadows. In their paper the authors write that “The orientation of the bicone relative to the galactic plane suggests a striking possibility: if the excess light at bright angles results from reflection of AGN emission off of diffuse galactic dust, then the dark rays could be the projected shadow of nuclear dust (possibly the torus), cast by AGN light.”

They add: “The dark and light pattern, therefore, is analogous to “crepuscular rays” seen in the form of sunbeam patterns cast by clouds when the Sun is obscured by the horizon.”

The torus acts as a kind of funnel, directing in-falling material toward the black hole itself. If Maksym and his colleagues are correct, it means that the torus itself is thin, and has gaps in it. It wouldn’t be the first one.

Other observations with ‘scopes like the Chandra X-ray Observatory found similar toruses around other black holes that leaked x-rays through holes. The question of why there are holes in the torus is so far unanswered, but it could be because of the SMBH’s powerful gravity warping the torus and creating gaps.

“It’s possible that the warping creates big enough gaps for some of the light to shine through, and as the torus rotates, beams of light could sweep across the galaxy like lighthouse beams through fog,” Maksym added.

These simple, cartoon-like images from the study illustrate four possible explanations for the light rays coming from IC 5063. The researchers consider the Shadow of a Warped Torus the most likely explanation. Image Credit: Maksym et al, 2020.

Modern observatories like the Hubble create an enormous amount of data and images. Sometimes more than scientists can handle. The scientific interest in this image was spurred on by a dedicated citizen scientist named Judy Schmidt.

Schmidt is an amateur astronomer and artist in Modesto, California. She prowls through the Hubble archives looking for interesting images to work with, which she then shares on Twitter (https://twitter.com/SpaceGeck). Maksym is one of her followers, and when she posted this image of the galaxy with its crepuscular rays in December 2019, it caught his interest.

Schmidt is particularly interested in galaxies like IC 5063 with active cores. The light beams weren’t visible when she initially saw the image. They only appeared when she began to work with the image and process it. “I had no idea they were there, and even after I’d processed it, I kept blinking my eyes wondering if I was seeing what I thought I was seeing,” Schmidt said.

“It was something I’d never seen before, and even though I had strong suspicions about them being shadow rays or ‘crepuscular rays,’ as Peter has dubbed them, it’s easy to let one’s imagination and wishful thinking run wild,” she explained. “I figured if I was wrong, someone would come to ground me.”

Schmidt’s Twitter followers engaged in a lively debate on the nature of the light rays. That’s when Maksym stepped in. He had already been studying Hubble images of black hole jets, so he decided to engage with Schmidt’s image professionally and write a paper.

So the venerable Hubble Space Telescope, as aged as it is, was put to use. Despite the Hubble being upstaged by more modern and powerful telescopes in recent years, it still has keen eyes. It used its Wide Field Camera 3 and Advanced Camera for Surveys to peer into IC 5063 in the near-infrared. Near-infrared light pierces through the dust to reveal more detail of the structure of the galaxy.

STS61 was the first servicing mission to The Hubble Space Telescope being serviced by the STS61 mission. The Hubble keeps going, but it won’t be serviced again. Sad face. Credit: NASA

“Older images from telescopes on the ground showed maybe hints of this kind of structure, but the galaxy itself is such a mess that you’d never guess that this is what’s going on without Hubble,” Maksym explained. “Hubble has sharp pictures, is sensitive to faint things, and has a big enough field of view to image the entire galaxy.”

Maksym plans to continue to study IC 5063, and he hopes that more researchers will work to try to understand it, too.

“We will want to keep investigating, and it will be great if other scientists try to test our conclusions, too, with new observations and modeling,” he said. “This is a project that is just begging for new data because it raises more questions than it answers.”

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Evan Gough

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