Free Astronomy Magazine January-February 2020

52 JANUARY-FEBRUARY 2020 SPACE CHRONICLES This new ALMA dis- covery could pro- vide a clue. “Counter-rotating gas streams are un- stable, which means that clouds fall into the black hole faster than they do in a disk with a sin- gle rotation direc- tion,” said Impel- lizzeri. “This could be a way in which a black hole can grow rapidly.” NGC 1068 (also known as Messier 77) is a spiral galaxy approximately 47 million light-years from Earth in the direction of the constellation Cetus. At its center is an active galactic nu- cleus, a supermas- sive black hole that is actively feeding itself from a thin, rotating disk of gas and dust, also known as an accre- tion disk. Previous ALMA observations re- vealed that the black hole is gulping down material and spewing out gas at incredibly high speeds. This gas that gets expelled from the accre- tion disk likely contributes to hiding the region around the black hole from optical telescopes. Impellizzeri and her team used ALMA’s superior zoom lens abil- ity to observe the molecular gas around the black hole. Unexpect- edly, they found two counter-rotat- ing disks of gas. The inner disk spans 2-4 light-years and follows the rotation of the galaxy, whereas the outer disk (also known as the torus) spans 4-22 light-years and is rotating the opposite way. A LMA image showing two disks of gas moving in opposite directions around the black hole in galaxy NGC 1068. 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. [ALMA (ESO/NAOJ/NRAO), V. Impellizzeri; NRAO/AUI/NSF, S. Dagnello] “We did not expect to see this, be- cause gas falling into a black hole would normally spin around it in only one direction,” said Impellizzeri. “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 cen- ters,” explained co-author Jack Gallimore from Bucknell University in Lewisburg, Pennsylvania. “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 as- tronomers think that the backward flow in NGC 1068 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 ap- pears 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 hap- pen after a few orbits or a few hun- dred thousand years. The rotating streams of gas will collide and be- come unstable, and the disks will likely collapse in a luminous event as the molecular gas falls into the black hole. Unfortunately, we will not be there to witness the fire- works,” said Gallimore. !