While we’re all partial to a sweet treat, astronomers are particularly excited about a newly spotted celestial doughnut tucked away in the centre of a nearby galaxy. The enormous ring-shaped dust cloud is hiding a supermassive black hole, a finding that confirms a 30-year-old theory about active galactic nuclei – some of the brightest and most enigmatic objects in the universe.
Reporting their findings in Nature, the international team of researchers say that their observations of the dense, obscuring dust cloud around the black hole provide confirmation of a theory known as the Unified Model of Active Galactic Nuclei (AGN).
AGNs are extremely energetic regions powered by supermassive black holes and are found at the centre of some galaxies. These black holes feed on large volumes of cosmic dust and gas. As this material spirals into the black hole’s inescapable maw, it releases so much energy that it often outshines all the stars in the galaxy.
These dazzlingly bright objects were first spotted in the 1950s, and astronomers have been curious about them ever since. They long ago observed that, like any good doughnut, AGNs come in different “flavours” – some release bursts of radio waves while others don’t; some shine brightly in visible light, while others are more subdued.
This initially led astronomers to classify AGNs into two distinct categories: type 1 and type 2 objects. However, the Unified Model that was developed 30 years ago states that, despite their differences, all AGNs have the same basic structure: a supermassive black hole surrounded by a thick ring of dust. The theory says that any observed differences are simply the result of the angle at which we view the black hole and its ring from Earth.
Astronomers had found some evidence to support the Unified Model before, including spotting warm dust at the centre of the nearby galaxy Messier 77, also known as NGC 1068. 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.
Now, thanks to extraordinarily detailed new observations of the centre of Messier 77, the theory has finally been confirmed.
“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 study author Dr Violeta Gámez Rosas.
“Whilst no single result will settle all the questions we have, we have taken a major step in understanding how AGNs work.”
The observations were made possible thanks to an instrument called the Multi AperTure mid-Infrared SpectroScopic Experiment (MATISSE) mounted on ESO’s VLTI, located in Chile’s Atacama Desert. MATISSE combines the light of all four of ESO telescopes using a technique called interferometry. The team used MATISSE to scan the centre of Messier 77, located 47 million light-years away in the constellation Cetus, revealing its torus-shaped dust cloud and helping to define its properties.
“MATISSE can see a broad range of infrared wavelengths, which lets us see through the dust and accurately measure temperatures,” says co-author Walter Jaffe, a professor at Leiden University, in the Netherlands.
“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.”
The team were able to build a detailed picture of the dust cloud, and pinpointed the location of the black hole hidden inside. Each of their observations agreed with predictions made by the Unified Model.
The researchers say their results do more than add to our understanding of AGNs. Gámez Rosas says the findings could also help us “better understand the history of the Milky Way, which harbours a supermassive black hole at its centre 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.