A global research team under the direction of Takuma Izumi, an assistant
professor at the National Astronomical Observatory of Japan, has made
significant progress in using the Atacama Large Millimeter/submillimeter
Array (ALMA) to observe the close-by active galactic nucleus of the Circinus
Galaxy at an exceptionally high resolution (roughly 1 light-year).
The findings of this discovery are reported in Science under the title
"Supermassive black hole feeding and feedback observed on sub-parsec
scales."
This is the first quantitative measurement of gas fluxes and their
structures in the close proximity of a supermassive black hole in all phase
gases, including plasma, atomic, and molecular, down to a few light-years.
Consequently, the accretion flow towards the supermassive black hole has
been clearly caught by the researchers, and it has been discovered that this
accretion flow is produced by a physical phenomenon called "gravitational
instability."
Moreover, the group discovered that a large fraction of this accretion flux
is not used in the black hole's formation. in an alternative, the majority
of the gas is ejected from the black hole's vicinity in atomic or molecule
outflows and then recycles back into the gas disk to take part in another
accretion flow in the direction of the black hole. This process is similar
to a water fountain. These results provide an important step towards a
thorough understanding of supermassive black hole formation processes.
"Supermassive black holes" are black holes with masses greater than a
million times that of the sun that are located at the core of many massive
galaxies. These gigantic black holes: how do they form? A key growth
mechanism that has been suggested by earlier studies is "gas accretion" onto
the black hole. This is the mechanism via which gas in the host galaxy
gravitates in the direction of the black hole in the center.
The supermassive black hole's gravity accelerates gas that congregates
extremely near to it at extraordinary speeds. This gas warms up to several
million degrees and generates bright light as a result of strong friction
between the particles. An active galactic nucleus (AGN) is this phenomena,
and it can occasionally be brighter than the galaxy's total starlight. It's
interesting to note that outflows are believed to result from the massive
energy of this active galactic center blowing away some of the gas that
falls towards the black hole (accretion flow).
In-depth understandings of gas accretion mechanisms from the 100,000
light-year scale of the galaxies down to a few hundred light-years at the
core have been obtained via both theoretical and observational
investigations. But because of its extremely tiny spatial scale, the gas
accretion within a considerably smaller region—especially within a few dozen
light-years from the galactic center—has remained elusive.
Measurements of the accretion flow rate (the quantity of gas streaming in)
and the volumes and types of gases (plasma, atomic gas, molecular gas) that
are released as outflows at that tiny scale, for example, are required to
quantitatively understand the formation of black holes. Sadly, until lately,
observational understanding in this area has not advanced all that
much.
In this work, the accretion flow towards the supermassive black hole within
the high-density gas disk that spans many light-years from the galactic
center was first successfully captured by the research team. Because of the
region's tiny size and the intricate movements of gas close to the galactic
center, identifying this accretion flow has long been a difficult
undertaking.
In this case, however, the researchers identified the precise area where
light from the background brightly flashing active galactic center was being
absorbed by the foreground molecular cloud. The high-resolution observations
with ALMA enabled this identification. This absorbing element is traveling
away from us, according to a thorough examination. The fact that there is
constantly absorbing material between us and the active galactic nucleus
suggests that the team has been effective in capturing the flow of accretion
towards the nucleus.
Moreover, the scientific group has also clarified the physical process that
causes this gas accumulation. The gravitational pull of the detected gas
disk was so great that the pressure derived from the disk's velocity was
unable to support it.
This leads to the collapse of the gas disk under its own weight, resulting
in the formation of intricate structures and the loss of its ability to
sustain steady motion near the galactic center. Consequently, the gas
descends quickly in the direction of the center black hole. The physical
phenomena at the center of the galaxy known as "gravitational instability"
has now been vividly exposed by ALMA.
Furthermore, the quantitative knowledge of gas movements surrounding the
active galactic nucleus has been greatly increased by this work. The
accretion rate at which gas is fed into the black hole may be computed using
the density of the observed gas and the velocity of the accretion flow. This
rate was discovered to be surprisingly large, thirty times more than what is
really needed to maintain the activity of this active galactic
nucleus.
Stated differently, the black hole was not growing because most of the
accretion flux surrounding the galactic center, at the 1-light-year scale,
was not contributing to its formation. So where did all of this extra gas
go? This work has also solved this mystery: ALMA discovered outflows from
the active galactic nucleus from high-sensitivity measurements of all phase
gases (medium-density molecular, atomic, and plasma; red, blue, and pink
areas in the first figure above).
It was discovered by quantitative research that most of the gas that was
directed into the black hole was released as discharges of molecules or
atoms. They ultimately made their way back to the gas disk since they were
unable to escape the black hole's gravitational pull because of their
sluggish velocity. There, they completed an intriguing gas recycling process
in the galactic center by being recycled back into an accretion flow towards
the black hole, much like a fountain (second image).
In terms of this research's accomplishments, Izumi states, "Detecting
accretion flows and outflows in a region just a few light-years around the
actively growing supermassive black hole, particularly in a multiphase gas,
and even deciphering the accretion mechanism itself, are indeed monumental
achievements in the history of supermassive black hole research."
He goes on, "We need to investigate various types of supermassive black
holes that are located farther away from us in order to fully understand the
growth of supermassive black holes in cosmic history." We have great hopes
for the continued use of ALMA and for planned big radio interferometers in
the next generation, as this calls for high-resolution and high-sensitivity
observations."
