Supermassive black hole caught hiding in a ring of cosmic dust

(Nanowerk News) Active Galactic Nuclei (AGNs) are extremely energetic sources powered by supermassive black holes and found at the centre of certain galaxies. These black holes are fed by large volumes of cosmic dust and gas. 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 the Very Large Telescope Interferometer (VLTI) of the European Southern Observatory (ESO), a team of researchers led by Violeta Gámez Rosas, from Leiden University in the Netherlands, have taken a key step towards understanding how they work and what they look like up close.
The results are published in Nature ("Thermal imaging of dust hiding the black hole in the Active Galaxy NGC 1068").
galaxy NGC 1068
The left panel of this image shows a dazzling view of the active galaxy NGC 1068 captured with the FOcal Reducer and low dispersion Spectrograph 2 (FORS2) instrument on ESO’s Very Large Telescope (VLT). The right panel shows a blow-up view of the very inner region of this galaxy, its active galactic nucleus, as seen with the MATISSE instrument on ESO’s Very Large Telescope Interferometer (VLTI) at infrared wavelengths. (Image: ESO/Jaffe, Gámez-Rosas et al.)
By making extraordinarily detailed observations of the centre of the galaxy NGC 1068 (also known as Messier 77 or M77), at infrared wavelengths in the range of 3 to 12 micrometre, 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 theory known as the Unified Model of AGNs.
Astronomers know there are different types of AGN. Certain AGNs shine brightly in visible light, while others, like NGC 1068, 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 centre of NGC 1068. However, doubts remained about whether this dust could fully hide a black hole and, hence explain why this AGN shines less brightly in visible light than others.
“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. While no single result will settle all the questions we have, we have taken a major step in understanding how AGNs work”, says Gámez Rosas. “Our results should lead to a better understanding of the inner workings of AGNs. They 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 observations were made possible thanks to the Multi AperTure mid-Infrared SpectroScopic Experiment (MATISSE) mounted on the VLTI, located on Cerro Paranal 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 centre of NGC 1068, located 47 million light years away in the constellation Cetus.
“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 NGC 1068. 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.
Combining the dust temperature changes (from around room temperature to about 1200 oC) 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 with a more extended disc — with the black hole positioned at its centre 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.
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.
Team member Bruno Lopez, the MATISSE Principal Investigator at the Observatoire de la Côte d’Azur in Nice, France, says: "NGC 1068 is an important prototype AGN and a wonderful motivation to expand our observing programme and to optimise MATISSE to tackle a wider sample of AGNs."
“Sophisticated data processing methods were developed for the reconstruction of the images, which are the very first high-resolution images of NGC 1068 derived at long infrared wavelengths in the range from 3 to 12 micrometre, where the dust radiation is strongest", explains Karl-Heinz Hofmann from the Max Planck Institute for Radio Astronomy (MPIfR) in Bonn.
“Future observations with the MATISSE instrument with low and additionally high spectral resolution will allow us to study the structure of AGNs at various important wavelengths”, concludes Gerd Weigelt, also from MPIfR. ”Combined with interferometric observations at radio wavelengths and other complementary measurements, the observations will enable the investigation of the inner AGN regions with unprecedented precision and improve our understanding of the physics of AGNs.”
Source: Max Planck Institute for Radio Astronomy
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