Rotating gaseous donut around an active supermassive black hole

The central region of the spiral galaxy M77. 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 indicates emission from formyl
ions (HCO+) and green indicates hydrogen cyanide emission.
Credit: ALMA (ESO/NAOJ/NRAO), Imanishi et al., NASA/ESA
Hubble Space Telescope and A. van der Hoeven

High-resolution observations with the Atacama Large
Millimeter/submillimeter Array (ALMA) imaged a rotating dusty
gas torus around an active supermassive black hole. The
existence of such rotating donuts-shape structures was first
suggested decades ago, but this is the first time one has
been confirmed so clearly. This is an important step in
understanding the co-evolution of supermassive black holes
and their host galaxies.

Almost all hold concealed monstrous in their centers. Researchers have
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 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

Aiming to solve this shadowy problem, a team of astronomers
utilized the high resolution of ALMA to observe the center of
spiral galaxy M77. The central region of M77 is an “active
galactic nucleus,” or 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.

The team imaged the area around the supermassive black hole in
M77 and resolved a compact gaseous structure with a radius of
20 light-years. And, the astronomers 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 . This is called
the ‘unified model’ of AGN,” explained Masatoshi
Imanishi (National Astronomical Observatory of Japan), the lead
author on a paper published in the Astrophysical Journal
. “However, the dusty gaseous donut is very tiny in
appearance. With the high resolution of ALMA, now we can
directly see the structure.”

Many astronomers have observed the center of M77 before, but
never has the rotation of the gas donut around the black hole
been seen so clearly. Besides the superior resolution of ALMA,
the selection of molecular emission lines to observe was key to
revealing the structure. 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,” said Imanishi.

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 Milky Way Galaxy, where we live, also has a supermassive
black hole at its center. This black hole is, however, in a
very quiet state. Only a tiny amount of gas is accreting onto
it. Therefore, to investigate an AGN in detail, astronomers
need to observe the centers of distant galaxies. M77 is one of
the nearest AGN and a suitable object for peering into the very
center in detail.

Explore further:

Astronomers shed light on formation of black holes and

More information: Masatoshi Imanishi et al. ALMA Reveals
an Inhomogeneous Compact Rotating Dense Molecular Torus at the
NGC 1068 Nucleus, The Astrophysical Journal (2018).
DOI: 10.3847/2041-8213/aaa8df