High resolution observations using the Atacama Large Millimeter/submillimeter Array (ALMA) have revealed the presence of a rotating dusty gaseous torus around an active supermassive black hole at the center of the barred spiral galaxy Messier 77.
Artist’s impression of the dusty gaseous torus around an active supermassive black hole. Image credit: ALMA / ESO / NAOJ / NRAO.
Astronomers known for a long time that the more massive the galaxy is, the more massive the central black hole is.
This sounds reasonable at first, but host galaxies are 10 billion times bigger than the central black holes; it should be difficult for two objects of such vastly different scales to directly affect each other.
So how could such a relation develop? Aiming to solve this shadowy problem, Dr. Masatoshi Imanishi from the National Astronomical Observatory of Japan and co-authors utilized ALMA’s high resolution to observe the center of Messier 77, which is located approximately 47 million light years away.
Messier 77’s central region is an active galactic nucleus (AGN), which means that matter is vigorously falling toward the central supermassive black hole and emitting intense light.
AGNs can strongly affect the surrounding environment, therefore they are important objects for solving the mystery of the co-evolution of galaxies and black holes.
Dr. Imanishi and colleagues imaged the area around the supermassive black hole in Messier 77 and resolved a compact gaseous structure with a radius of 20 light-years. They found that the compact structure is rotating around the black hole, as expected.
“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,” Dr. Imanishi said.
“However, the dusty gaseous donut is very tiny in appearance. With the high resolution of ALMA, now we can directly see the structure.”
This composite image shows the central region of the barred spiral galaxy Messier 77. The NASA/ESA Hubble Space Telescope imaged the distribution of stars. ALMA revealed the distribution of gas in the very center of the galaxy. ALMA imaged a horseshoe-like structure with a radius of 700 light-years and a central compact component with a radius of 20 light-years. The latter is the gaseous torus around the AGN. Red color indicates emission from formyl ions and green indicates hydrogen cyanide emission. Image credit: ALMA / ESO / NAOJ / NRAO / Imanishi et al / NASA / ESA / Hubble / A. van der Hoeven.
The team observed specific microwave emission from hydrogen cyanide molecules (HCN) and formyl ions (HCO+).
These molecules emit microwaves only in dense gas, whereas the more frequently observed carbon monoxide (CO) emits microwaves under a variety of conditions.
The torus around the AGN is assumed to be very dense, and the team’s strategy was right on the mark.
“Previous observations have revealed the east-west elongation of the dusty gaseous torus. The dynamics revealed from our ALMA data agrees exactly with the expected rotational orientation of the torus,” Dr. Imanishi said.
Interestingly, the distribution of gas around the supermassive black hole is much more complicated than what a simple unified model suggests.
The torus seems to have an asymmetry and the rotation is not just following the gravity of the black hole but also contains highly random motion.
These facts could indicate the AGN had a violent history, possibly including a merger with a small galaxy. Nevertheless, the identification of the rotating torus is an important step.
The team’s findings appear in the Astrophysical Journal Letters.
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Masatoshi Imanishi et al. 2018. ALMA Reveals an Inhomogeneous Compact Rotating Dense Molecular Torus at the NGC 1068 Nucleus. ApJL 853, L25; doi: 10.3847/2041-8213/aaa8df
