Astronomers used the ALMA radio array in Chile to show that two narrow spiral arms are funneling gas toward the central monster in the Circinus galaxy. The galaxy sits about 13 million light years away in the southern sky.
Only a small share of that inflow reaches the inner engine. Less than 12 percent keeps going inward, while the rest gets pushed back out.
Circinus galaxy’s spiral arms
Circinus hosts a supermassive black hole, a central black hole with millions of Suns. Its gravity controls the region around the core.
ALMA can map cold gas with sharp detail at sizes far smaller than a light year. Lead researcher Wout M. Goesaert of Leiden University (LU) guided the new analysis.
Around the black hole sits an accretion disk, the hot, fast spinning gas that releases energy as it spirals inward. Surrounding that is the torus, a thick, doughnut-like ring of cooler dust and gas.
Encasing both is the circumnuclear disk, a wider gas disk around the galactic center on tens of light year scales. This outer structure can act as a reservoir that feeds the inner ring.
Feeding a black hole
In a new study the team traced dust and dense gas and found two spiral arms that connect straight into the torus of the Circinus galaxy. Those arms are the missing supply lines.
“If there is no supply channel, such a disk will continue to spin around at a decent distance forever. So you need a passageway of some kind,” said Goesaert.
The analysis estimates a feeding rate between about 0.3 and 7.5 Suns per year. That is plenty to power the core if most of it could reach the inner disk.
The geometry shows a layered structure. The molecular gas arms lie slightly wider than the dust and ionized components.
Spiral structures in the circumnuclear disk of the Circinus Galaxy’s black hole overlaid on the band 7 CO map. The dashed cyan arrow represents the proposed inflow through the NW hook that feeds the torus. The synthesized beam size is drawn in the bottom left as a filled ellipse, and the cyan cross indicates the AGN position. Click image to enlarge. Credit: arXivCircinus galaxy’s gas moves fast
Gas slides inward along the arms at speeds up to about 93,000 miles per hour. That is swift enough to cross one light year in roughly 11,000 years.
Closer in, the ionized wind appears to launch or become focused about 0.16 light years from the center. The opening angle differs on each side, which hints that the launch mechanism is not the same in all directions.
The measurements point to density differences rather than temperature differences between the two cone edges. That pattern matches a wind that starts or is shaped by a warped inner structure.
These details help place the handoff between feeding and feedback. They also show why the outflow looks lopsided.
Why most gas does not make it
A long-standing model proposes a radiation driven fountain, gas that rises from the core, cools, and falls back to thicken the torus. That cycling can stir turbulence and keep the ring puffy.
Direct observations in Circinus show molecular outflows that can carry between a fraction of a Sun and more than ten Suns per year. Those numbers can easily cancel most of the inflow.
The new feeding rates imply a short refilling time for the torus. The math points to roughly 120,000 years to 2.7 million years to load it up.
That timescale lines up with the stop and start behavior many active cores show. Activity may flicker as the supply from the outer disk waxes and wanes.
Watching Circinus galaxy’s gas move
Water maser signals, natural microwave lasers from molecules, mapped a thin, warped disk very close to the black hole years ago. That warped structure is well placed to steer or confine a wind.
Mid-infrared images from the MATISSE instrument traced a dusty cone that matches the ionized cone near the core. The cones share a similar axis but show subtle differences in width.
High resolution analysis of the ionized gas reveals a distinct forked shape in the outflow to one side. That feature looks like a wind that hits denser clumps and splits.
The new ALMA view links these pieces into a single flow chart. Gas travels inward along spiral arms, piles into the torus, then a portion is launched back out in a layered wind.
Astronomer Walt Goesaert mapped how molecular gas is distributed in the Circinus galaxy, about 13 million light-years away. The galaxy is shown in the top left corner in visible light. The two insets are images taken with the Atacama Large Millimeter/submillimeter Array (ALMA). Click image to enlarge. Credit: ESOWhat’s next for the Circinus galaxy?
The finding sets a clean test for galaxy models. Any realistic simulation should grow spiral arms in the inner disk and use them to move gas inward.
Future images with the Event Horizon Telescope and the Extremely Large Telescope could resolve the last step in the chain. Those facilities can probe where the wind begins and how radiation pushes on the gas.
Similar ALMA maps of other nearby active galaxies will show whether Circinus is special or typical. If the same two arm pattern appears elsewhere, the feeding rulebook may be simple.
The work also offers a practical way to estimate duty cycles. If the torus fills on a million year timescale, then on similar spans the core should brighten and fade.
The study is published in Astronomy & Astrophysics.
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