Supermassive black holes, millions to billions of times the mass of our Sun, are found at the centers of galaxies. Many of these enormous objects are hidden within a thick doughnut-shape cloud of dust and gas known as a torus. Using the Atacama Large Millimeter/submillimeter Array (ALMA), astronomers have found direct evidence that a torus around a supermassive black hole in the barred spiral galaxy Messier 77 consists of counter-rotating and misaligned disks.
An artist’s impression of the center of the barred spiral galaxy Messier 77. ALMA discovered two counter-rotating flows of gas around the black hole. The colors in this image represent the motion of the gas: blue is material moving toward us, red is moving away. Image credit: NRAO / AUI / NSF / S. Dagnello.
Messier 77, also known as NGC 1068, is a barred spiral galaxy located some 47 million light-years from Earth in the constellation Cetus.
At its center is an active galactic nucleus, a supermassive black hole that is actively feeding itself from an accretion disk.
Previous ALMA observations revealed that the black hole is not only gulping down material, but also spewing out gas at incredibly high speeds, up to one million mph (500 km/s), and is actually the source of its dusty torus of dust and gas.
Dr. Violette Impellizzeri of the National Radio Astronomy Observatory and colleagues used ALMA’s superior zoom lens ability to observe the molecular gas in the torus around the black hole.
“Thanks to the spectacular resolution of ALMA, we measured the movement of gas in the inner orbits around the black hole,” Dr. Impellizzeri said.
Unexpectedly, the astronomers found two counter-rotating disks of gas.
The inner disk stretches between 2 and 4 light-years from the black hole and follows the rotation of the galaxy, whereas the outer disk stretches between 4 and 22 light-years and is rotating the opposite way.
This ALMA image shows two disks of gas moving in opposite directions around the black hole in Messier 77. The colors in this image represent the motion of the gas: blue is material moving toward us, red is moving away. The white triangles are added to show the accelerated gas that is expelled from the inner disk, forming a thick, obscuring cloud around the black hole. Image credit: ALMA / ESO / NAOJ / NRAO / V. Impellizzeri / AUI / NSF / S. Dagnello.
“We did not expect to see this, because gas falling into a black hole would normally spin around it in only one direction,” Dr. Impellizzeri said.
“Something must have disturbed the flow, because it is impossible for a part of the disk to start rotating backward all on its own.”
“Counter-rotation is not an unusual phenomenon in space. We see it in galaxies, usually thousands of light-years away from their galactic centers,” said Dr. Jack Gallimore, an astronomer at Bucknell University.
“The counter-rotation always results from the collision or interaction between two galaxies. What makes this result remarkable is that we see it on a much smaller scale, tens of light-years instead of thousands from the central black hole.”
The researchers think that the backward flow in Messier 77 might be caused by gas clouds that fell out of the host galaxy, or by a small passing galaxy on a counter-rotating orbit captured in the disk.
At the moment, the outer disk appears to be in a stable orbit around the inner disk.
“That will change when the outer disk begins to fall onto the inner disk, which may happen after a few orbits or a few hundred thousand years,” Dr. Gallimore said.
“The rotating streams of gas will collide and become unstable, and the disks will likely collapse in a luminous event as the molecular gas falls into the black hole.”
The team’s paper was published in the Astrophysical Journal.
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C.M. Violette Impellizzeri et al. 2019. Counter-Rotation and High-velocity Outflow in the Parsec-Scale Molecular Torus of NGC 1068. ApJL 884, L28; doi: 10.3847/2041-8213/ab3c64
