Astronomers have detected a torus rotating around the supermassive black hole at the center of spiral galaxy Messier 77, collecting observations that may shed light on why these weird structures exist.

AGN Cartoon
This not-to-scale cartoon shows a supermassive black hole along with its retinue of gaseous disk, clouds, and jets, altogether known as an active galactic nucleus (AGN). The torus is key to explaining how the same object can appear differently from different angles. A quasar that appears brilliant when viewed face-on turns faint when viewed through its torus of dusty gas.
C.M. Urry and P. Padovani

From car tires to chocolate-frosted donuts to infinity scarves, the torus is a relatively common shape in modern life. But the same can’t be said for nature, where globular shapes and thin disks are far more typical.

That’s why the torus is one of the more striking features of active galactic nuclei (AGN). In AGN supermassive black holes are gorging on gas, which heats up until it’s radiating more brightly than the entire host galaxy. But decades of indirect data show that these luminous behemoths often hide behind a fat, dusty torus. To explain observations, the torus must cover half of the AGN’s sky, which means it must be roughly as fat as it is tall — somewhere around 3 light-years for both dimensions.

Such a structure ought to collapse into a disk within tens of thousands of years. Instead, they apparently stick around for 100 million years — and astronomers still don’t know why.

That’s beginning to change with observations of Messier 77 (M77, aka NGC 1068), a black hole 10 million times the mass of the Sun that’s partially hidden by its encircling torus. The Atacama Large Millimeter/submillimeter Array (ALMA), high in the Atacama Desert in Chile, can image the torus itself, capturing the rotating, outflowing, and turbulent motions of its gas.

ALMA images M77 / NGC 1068's central regions
ALMA captured dense gas (inset) at the center of spiral galaxy Messier 77 (background), as imaged by the Hubble Space Telescope. The horseshoe-like structure spans 1,400 light-years and the central spot — the AGN torus — is 40 light-years wide. In the inset, red indicates emission from formyl ions (HCO+) and green indicates emission from hydrogen cyanide (HCN) molecules.
Inset: ALMA (ESO/NAOJ/NRAO) / Imanishi et al. Background: NASA / ESA Hubble Space Telescope & A. van der Hoeven

In 2016 Santiago García-Burillo (National Astronomical Observatory of Madrid) and Jack Gallimore (Bucknell University) led teams reporting ALMA observations of M77 that peer into the galaxy’s core. Both teams used carbon monoxide molecules to trace vastly more abundant (but more difficult to observe) molecular hydrogen gas. While Garcia-Burillo’s team captured turbulence in the torus, Gallimore’s group caught an outflow, presumably coming from the accretion disk, that’s blowing into (and puffing up) the torus.

Now, Masatoshi Imanishi (National Astronomical Observatory of Japan) and colleagues have imaged M77’s torus in even greater detail, with images twice as sharp as previous ones. The results appear in Astrophysical Journal Letters.

The team measured hydrogen cyanide — while it’s a poisonous acid on Earth, it serves as a tracer of dense hydrogen gas in space. Because their observations follow denser gas, they’re able to capture the rotational motions of the torus. The 40 light-year-wide donut orbits the supermassive black hole at roughly 10 kilometers per second (20,000 mph).

The torus at the center of M77 / NGC 1068
This image shows gas moving around M77's supermassive black hole. The gas moving toward us is shown in blue and that moving away from us is in red. The gas’s rotation is centered around the black hole (unseen).
ALMA (ESO / NAOJ / NRAO) / Imanishi et al.

However, further analysis indicates that there’s a lot more to the torus than rotation — their observations are consistent with the previous results showing other motions (turbulence and outflows) that sustain the torus.

“ALMA is doing a fantastic job exposing the secrets of NGC 1068, and the observations are only getting better,” Gallimore says.


Image of Anthony Barreiro

Anthony Barreiro

February 24, 2018 at 12:11 am

At 9th magnitude and with a bright condensed core, M77, near the head of Cetus the sea monster, is easily observed in a small amateur telescope. The galaxy is currently getting low in the southeast at nightfall for observers at mid-northern latitudes, but it's still observable in a reasonably dark sky. Or you may want to wait until next Fall and early Winter to see it highest in the south.

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Image of Anthony Barreiro

Anthony Barreiro

February 26, 2018 at 4:02 pm

Whoops. The galaxy is getting low in the southwest at nightfall for observers at mid-northern latitudes.

I often get left and right, and east and west, backwards. I do better with north and south and up and down.

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Image of OldDogZeroOne


February 24, 2018 at 10:06 am

Just a thought on the uncommon torus in nature. Think about smoke rings, they form at explosive volcanic vents at times when there is an OUTFLOW of high speed gas. Perhaps the vector of rotation observed is around the center of the donut's body beginning tangent to the outflow and arcing out and back to form a donut, rather than a disk, where the primary rotation would be at right angles to the outflow. Maybe the gas flows out and returns on both sides of this torus, driven by radiation from the AGN? Why would the gas turn back? Could there be a thin ring of dark matter rotating around the AGN, inside the torus forming a "core"? Can dark matter form structures on that scale? Looks like a field of future study.

Frank J. Cernik

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