Rotation observed of a ring of gas and dust surrounding a supermassive black hole

M77's active galactic nucleus with the gas moving in the inset (Image ALMA (ESO/NAOJ/NRAO), Imanishi et al.)
M77’s active galactic nucleus with the gas moving in the inset (Image ALMA (ESO/NAOJ/NRAO), Imanishi et al.)

An article published in the journal “Astrophysical Journal Letter” describes the best observation ever made of a ring of gas and dust surrounding a supermassive black hole. A team of astronomers used the ALMA radio telescope to observe the active galactic nucleus (AGN) of the M77 spiral galaxy that emits the intense electromagnetic radiation detected. This is the definitive proof of what was initially proposed as a theoretical concept for which increasingly clearer evidence was collected over time up to that presented in this research.

The existence of rotating rings – but technically the shape is called torus – of gas and dust was theorized already decades ago, when the concept of black hole was theoretical as well. The construction of more powerful and more sensitive telescopes allowed to investigate those phenomena with increasing accuracy.

In the case of active galactic nuclei fed by supermassive black holes, the problem was not only the distance of millions if not billions of light years but in some cases also the presence of dust and gas clouds that can obscure part of the electromagnetic frequencies making a nucleus invisible to certain telescopes despite its activity.

Radio telescopes can detect electromagnetic frequencies that can pass through dust and gas clouds, so their contribution to the investigation of supermassive black holes has been crucial. The ALMA (Atacama Large Millimeter/submillimeter Array) radio telescope, inaugurated in March 2013, allowed significant progress in various types of investigations, in this case thanks to direct observations of an active galactic nucleus.

The M77 galaxy, also known as NGC 1068, UGC 2188, PGC 10266 or Arp 37, is a spiral galaxy about 47 million light years from Earth. It’s a Seyfert galaxy, a class characterized by an active galactic nucleus whose spectrum shows strongly ionized gas. It’s one of the galaxies whose nucleus is obscured at visible light wavelengths due to the presence of dust and gas so it’s been studied over time thanks to its radio and microwave emissions.

The active galactic nucleus of M77 has been observed many times with various instruments, but this is the first time the ring of gas and dust’s rotation was observed around the supermassive black hole. To achieve this result, not only the power of the ALMA radio telescope was exploited, but also its sensitivity in detecting the molecules inside the ring.

The researchers observed the chemical “signature” of molecules such as hydrogen cyanide and formyl ions. They only emit microwaves in dense gas whereas carbon monoxide, which is more commonly detected, emits them in a variety of conditions. This suggests that the gas that makes the ring up is very dense.

The composition of that ring seems more complex than what is expected by the current models. The detections indicate an asymmetry and a rotation that isn’t limited to following the supermassive black hole’s gravity but includes random movements. This suggests a violent history, which could for example be the consequence of a galactic fusion.

A research topic concerns the relationship between supermassive black holes and the galaxies that host them. This includes the relationship between their masses and the influence that the black hole can have on star formation. For this reason the ALMA radio telescope offers valuable information on the chaotic situation around it.

The galaxy M77 with its active galactic nucleus in the inset (Image ALMA (ESO/NAOJ/NRAO), Imanishi et al., NASA/ESA Hubble Space Telescope and A. van der Hoeven)
The galaxy M77 with its active galactic nucleus in the inset (Image ALMA (ESO/NAOJ/NRAO), Imanishi et al., NASA/ESA Hubble Space Telescope and A. van der Hoeven)

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