4 mins read 22 Feb 2022

Cosmic Dust Confirms Active Galactic Nuclei Model

Astronomers have found evidence that supports the theory that all active galactic nuclei (AGN) have the same underlying structure and only appear to have different properties because of their orientation relative to Earth. 

Left: Active galaxy Messier 77 captured with the Focal Reducer and low dispersion Spectrograph 2 (FORS2) instrument on ESO’s Very Large Telescope. Right: Close-up view of the inner region of this galaxy, its active galactic nucleus, as seen with the MATISSE instrument on ESO’s Very Large Telescope Interferometer. Credit ESO/Jaffe, Gámez Rosas et al.

An international collaboration of astronomers from Sydney to Santiago have used the Very Large Telescope Interferometer (VLTI) at the European Southern Observatory (ESO) facility atop Cerro Paranal, Chile to observe and analyse the light coming from the centre of NGC 1068 - a spiral galaxy nearly 47 million light-years away.

The group, led by Violeta Gámez Rosas from Leiden University in the Netherlands, found a thick ring of cosmic gas and dust obscuring the supermassive black hole (SMBH) at the centre of NGC 1068 - also known as Messier 77 -  which is understood to be the source of material powering the energetic jets that flow outwards from the SMBH.

“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,” Gámez Rosas explained.

Active black holes come in a range of flavours including quasars, blazars, and Seyfert galaxies. While each of these astronomical phenomena have somewhat different observed properties, they all contain a central SMBH surrounded by a disk of infalling material. 

In order to accurately measure the central region of Messier 77, the team used the VLTI’s GRAVITY instrument which combines all four VLT telescope beams into high-resolution data. The images, which were composed of infrared light, were then calibrated using a software tool designed by Anthony Soulain, a researcher at the Sydney Institute for Astronomy at the University of Sydney.  

The Unified Model of Active Galactic Nuclei

This infographic shows a simplified version of the Unified Model of AGN which proposes that any apparent difference between AGN is due to the orientation of their SMBH and its dust region, which could obstruct or completely hide the black hole from our perspective. Credit: ESO/L. Calçada and M. Kornmesser.

Though some AGN shine most brightly at visible wavelengths and others produce exceptionally luminous bursts of radio light, the unified model of AGN - originally developed by astrophysicist Robert Antonucci of the University of California, Santa Barbara in 1993 - suggests that these observed differences stem from variations of the host galaxy orientation with respect to our line of sight from Earth.  

Gámez Rosas noted that “whilst no single result will settle all the questions we have, we have taken a major step in understanding how AGNs work.”

Astronomers often have to carefully model the shapes and properties of the objects they observe because we are generally limited, here on Earth, to seeing two-dimensional projections of three-dimensional objects. Constellations, for example, are almost always unrelated stars at varying distances moving in completely different directions; they just appear to make a recognisable shape in our sky.

According to Walter Jaffe, a professor at Leiden University and co-author of the study, these new observations were able to “detail the changes in temperature and absorption of the dust clouds around the black hole,” as the advanced Multi AperTure mid-Infrared SpectroScopic Experiment (MATISSE) instrument mounted on the ESO’s VLTI “can see a broad range of infrared wavelengths, which lets us see through the dust and accurately measure temperatures.”

“We can celebrate the fact that Gámez Rosas and colleagues’ study is the best proof yet that the unified model is correct,” said Antonucci.

This work, published recently in Nature, enabled the astronomers to pinpoint the location of the SMBH at the core of NGC 1068 and “should lead to a better understanding of the inner workings of AGN” as Gámez Rosas summarised. “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.”  

This image from the Digitized Sky Survey (DSS) shows spiral galaxy Messier 77 and its surroundings. Messier 77 appears at the centre with the edge-on galaxy NGC 1055 to its upper-right. Credit: NASA/ESA, DSS 2.

The paper is available in the journal, Nature