ALMA Snaps Incredible Image Of Gas Ring Around Supermassive Black Hole

A composite image showing M77, with an insert showing the ALMA observation of material from the central region and the torus (orange blob within the green ring). ALMA (ESO/NAOJ/NRAO), Imanishi et al.

The Atacama Large Millimeter/submillimeter Array (ALMA) has taken a spectacular image of the ring of dust and gas surrounding the supermassive black hole inside galaxy M77. Measuring 40 light-years across, the structure is consistent with theoretical expectations and builds upon previous observations.

The ring is technically known as a dusty torus, a donut-like shape. As reported in the Astrophysical Journal Letters, the torus is compact and dense. Thanks to the high-caliber capabilities of ALMA, the researchers were able to track down the distribution of gas in exquisite detail and show for the first time that the ring is indeed rotating.

“Previous observations have revealed the east-west elongation of the dusty gaseous torus," lead author Masatoshi Imanishi, from the National Astronomical Observatory of Japan, said in a statement. "The dynamics revealed from our ALMA data agrees exactly with the expected rotational orientation of the torus.” 

The dusty torus is on the left with the insert showing its Doppler shift. The blue area represents gas moving towards us, while the red is gas moving away. ALMA (ESO/NAOJ/NRAO), Imanishi et al. 

The observations are really important for the study of active galactic nuclei (AGNs), the state of supermassive black holes can sometimes be in when they are feeding and emitting jets and light. The dusty torus is key to explaining how these objects appear in our telescopes.

“To interpret various observational features of AGNs, astronomers have assumed rotating donut-like structures of dusty gas around active supermassive black holes. This is called the ‘unified model’ of AGN,” explained Imanishi. “However, the dusty gaseous donut is very tiny in appearance. With the high resolution of ALMA, now we can directly see the structure.”

The distribution of gas around this particular AGN is not as simple as the unified model seemed to suggest. The ring of material is not a perfect torus. It is asymmetric and has a complicated motion. It does follow the rotation of the supermassive black hole but it includes some random motion that the researchers can’t currently explain. The team suggested that a collision with a small galaxy or a more violent phase of the AGN in the past might justify the peculiar motion.

ALMA was able to track down two molecules found in dense gas, hydrogen cyanide (HCN), and formyl ions (HCO+). Their unique emission was crucial to track down the movement of the gas, an important step in better understanding the environment around supermassive black holes.

Artist’s impression of a gaseous torus. ALMA (ESO/NAOJ/NRAO)



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