Hidden Supermassive AGN Among the Brightest and Most Enigmatic Objects in the Universe 

Galaxy Messier 77 AGN

 

The European Southern Observatory’s Very Large Telescope Interferometer (ESO’s VLTI) on the Paranal Mountain in the Atacama Desert of northern Chile has observed a cloud of cosmic dust at the center of the galaxy Messier 77 that is hiding a supermassive black hole. The findings have confirmed predictions made around 30 years ago and are giving astronomers new insight into “active galactic nuclei (AGN)” –some of the brightest and most enigmatic objects in the universe.

Unveiling the History of the Milky Way

Astronomers suggest that these enigmatic objects could help unveil the history of the Milky Way. It is generally accepted that all galaxies have a super massive black hole at their centers and that they sometimes eat more or less material, so that they can become active (or AGNs) for periods of time, and then become quiet for longer periods of time, astrophysicist Violeta Gámez Rosas at Leiden University in the Netherlands told The Daily Galaxy. “In the case of the Milky Way,”she explained, “there are signatures that tell us that maybe our galaxy was an AGN that has become quiet. So understanding AGNs better might help to disentangle if the Milky Way was or not an AGN before.”

Extreme Physics of AGNs

Active galactic nuclei (AGNs) are extremely energetic sources powered by supermassive black holes and found at the center of some galaxies. These black holes feed on large volumes of cosmic dust and gas. Before it is eaten up, this material spirals towards the black hole and huge amounts of energy are released in the process, often outshining all the stars in the galaxy.

Astronomers have been curious about AGNs ever since they first spotted these bright objects in the 1950s. Now, thanks to ESO’s VLTI, a team of researchers, led by Gámez Rosas, have taken a key step towards understanding how they work and what they look like up close.

New Insights Into the Physics of Supermassive Black Holes

Hidden Black Hole of Galaxy Messier 77

By making extraordinarily detailed observations of the center of the galaxy Messier 77, also known as NGC 1068, Gámez Rosas and her team detected a thick ring of cosmic dust and gas hiding a supermassive black hole. This discovery provides vital evidence to support a 30-year-old theory known as the Unified Model of AGNs.

Towards a Unified Model

Astronomers know there are different types of AGN. For example, some release bursts of radio waves while others don’t; certain AGNs shine brightly in visible light, while others, like Messier 77, are more subdued. The Unified Model states that despite their differences, all AGNs have the same basic structure: a supermassive black hole surrounded by a thick ring of dust.

According to this model, any difference in appearance between AGNs results from the orientation at which we view the black hole and its thick ring from Earth. The type of AGN we see depends on how much the ring obscures the black hole from our view point, completely hiding it in some cases.

Astronomers had found some evidence to support the Unified Model before, including spotting warm dust at the center of Messier 77. However, doubts remained about whether this dust could completely hide a black hole and hence explain why this AGN shines less brightly in visible light than others.

Real Nature of Dust Clouds and Their Role

“The real nature of the dust clouds and their role in both feeding the black hole and determining how it looks when viewed from Earth have been central questions in AGN studies over the last three decades,” explains Gámez Rosas. “Whilst no single result will settle all the questions we have, we have taken a major step in understanding how AGNs work.”

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The Unanswered Questions

When asked by The Daily Galaxy to expand on some of the key unanswered questions Gámez Rosas replied in an email: “Why is there so much asymmetry, and what is causing it? Why does the synchrotron emission have the similar morphology as the structures of the cold dust. Where is all the dust coming from? How is the dust in the polar regions getting there; by dusty winds? Or, is it generated in situ? What kind of dust is there in the regions closer to the black hole?

The observations were made possible thanks to the Multi AperTure mid-Infrared SpectroScopic Experiment (MATISSE) mounted on ESO’s VLTI, located in Chile’s Atacama Desert. MATISSE combined infrared light collected by all four 8.2-metre telescopes of ESO’s Very Large Telescope (VLT) using a technique called interferometry. The team used MATISSE to scan the center of Messier 77, located 47 million light-years away in the constellation Cetus.

MATISSE Peers Through the Dust

“MATISSE can see a broad range of infrared wavelengths, which lets us see through the dust and accurately measure temperatures. Because the VLTI is in fact a very large interferometer, we have the resolution to see what’s going on even in galaxies as far away as Messier 77. The images we obtained detail the changes in temperature and absorption of the dust clouds around the black hole,” says co-author Walter Jaffe, a professor at Leiden University.

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Pinpointed Where the Black Hole Must Lie

Combining the changes in dust temperature (from around room temperature to about 1200 °C) caused by the intense radiation from the black hole with the absorption maps, the team built up a detailed picture of the dust and pinpointed where the black hole must lie. The dust — in a thick inner ring and a more extended disc — with the black hole positioned at its center supports the Unified Model. The team also used data from the Atacama Large Millimeter/submillimeter Array, co-owned by ESO, and the National Radio Astronomy Observatory’s Very Long Baseline Array to construct their picture.

“Our results should lead to a better understanding of the inner workings of AGNs,” concludes Gámez Rosas. “They could also help us better understand the history of the Milky Way, which harbors a supermassive black hole at its center that may have been active in the past.”  

The researchers are now looking to use ESO’s VLTI to find more supporting evidence of the Unified Model of AGNs by considering a larger sample of galaxies.

ESO’s Extremely Large Telescope (ELT), set to begin observing later this decade, will also aid the search, providing results that will complement the team’s findings and allow them to explore the interaction between AGNs and galaxies.

Source: This research was presented in the paper “Thermal imaging of dust hiding the black hole in the Active Galaxy NGC 1068” (doi: 10.1038/s41586-021-04311-7) to appear in Nature.

Maxwell Moe, astrophysicist, NASA Einstein Fellow, University of Arizona via Leiden UniversityNature and ESO

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