World-first observation of lightyear-scale black hole gas flows

Supermassive blackholes sucking galactic gas towards their centre end up throwing most of it away, Japanese researchers have found in research they describe as “monumental.”

In imagery that evokes a baby getting most of its food over its face, bib and arms, rather than in its mouth, these massive gravity wells scattered throughout the universe have been found to use only a tiny amount of the gas drawn towards their centres.

The discovery was made in a detailed study of the Circinus constellation, about 13 million light-years away, with the Atacama Large Millimeter/submillimeter Array (ALMA) telescope.

At the heart of Circinus are two black holes – one supermassive, which is actively ‘feeding’ on nearby gas from the surrounding galaxy.

Under the black hole’s intense gravitational pull, these gases reach incredible speed, causing particles to collide, heat up, and emit light so intense it can be detected by telescopes millions of light years away.

Supermassive black holes live to a “reuse, recycle” mantra

Now, for the first time, the team led by National Astronomical Observatory of Japan assistant professor Takuma Izumi has measured the nature and behaviour of gas around these black hole events – known as active galactic nuclei – at a tiny, one-lightyear scale.

Black hole 1
The central region of the Circinus Galaxy observed with ALMA. The distributions of carbon monoxide (CO; reflecting the presence of medium-density molecular gas), atomic carbon (C; reflecting the presence of the atomic gas), hydrogen cyanide (HCN; reflecting the presence of high density molecular gas), and the hydrogen recombination line (H36α; reflecting the presence of ionized gas), are shown in red, blue, green, and pink, respectively. There is an active galactic nucleus at the center. This galaxy is known to have a tilted structure from the outer to the inner regions, with the central region resembling a nearly edge-on disk. The size of the central dense gas disk (green) is approximately 6 light-years: this has been observed clearly thanks to the high resolution of ALMA (see the inset for the zoom-up view). The plasma outflow travels almost perpendicular to the central dense disk. Credit: ALMA (ESO/NAOJ/NRAO), T. Izumi et al.

It’s a remarkable achievement considering such measurements have typically been at far less crisp resolutions, covering 100-100,000s of light years.

Their investigation captured the accretion of gas towards the centre of the black hole. Accretion discs – often imagined by space artists as a swirl of matter being sucked towards the black hole’s dark centre – are subject to immense gravitational forces, which causes them to collapse and the gas is then rapidly pulled to the centre.

Illustration showing interstellar gas being pulled in and expelled from a black hole
Illustration depicting the distribution of gas in Circinus’ active galactic nucleus. Credit: ALMA (ESO/NAOJ/NRAO), T. Izumi et al.

And the accretion rate that supplies gas to the black hole is about 30 times more than what is needed to grow the object. These calculations, along with their observations, show the surplus is instead spat back out as molecular or atomic gas, where it rejoins the accretion disc and the process begins again.

Izumi’s team likens this process to a water fountain, where water is spat out and caught in the basin below for reuse. However having only studied the phenomenon in relation to the Circinus galaxy, Izumi hopes to refine the study’s data by using ALMA to study more supermassive black holes.

“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,” Izumi says.

“But to comprehensively understand the growth of supermassive black holes in cosmic history, we need to investigate various types of supermassive black holes that are located farther away from us.”

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