At the core of a massive galaxy a billion light years from Earth a supermassive black hole is powering the cosmic equivalent of a monumental fountain, drawing in vast stores of cold molecular gas and spraying them back out again in an ongoing cycle.
Astronomers have long theorised that fountains such as this continually recirculate a galaxy’s star-forming fuel. Now an international team of scientists have proven it using observations of a giant elliptical galaxy known as Abell 2597 taken from both the Atacama Large Millimeter/submillimeter Array (ALMA), and the Very Large Telescope’s Multi Unit Spectroscopic Explorer (MUSE).
It is the first clear and compelling evidence for the simultaneous infalling and outflow of gas driven by a supermassive black hole. The researchers report their observations in the latest issue of the Astrophysical Journal.
Astrophysicist Dr Stephen Hamer, from the University of Bath Department of Physics, is an author on the paper. He said: “These fountain-like cycles are a key component in our theory of galaxy evolution, acting as a maintenance system preventing galaxies from growing too large too fast. They are particularly important in massive galaxies such as this which show little change over the past billion years.
“This system is one of the first in which we find clear evidence of both infalling and outflowing gas which traces the full cycle, confirming it is actively undergoing this process. We were only able to achieve this thanks to the combination of two of the most powerful observing facilities available to us; ALMA and MUSE.”
Dr Grant Tremblay, an astrophysicist at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, and lead author on the paper said: “The supermassive black hole at the center of this giant galaxy acts like a mechanical ‘pump’ in a water fountain. This is one of the first systems in which we find clear evidence for both cold molecular gas inflow toward the black hole and outflow or uplift from the jets that the black hole launches.”
According to the researchers, this entire system operates via a self-regulating feedback loop. The infalling material provides power for the fountain as it “drains” toward the central black hole, like water entering the pump of a fountain. This infalling gas then causes the black hole to ignite with activity, launching high-velocity jets of super-heated material that shoot out of the galaxy. As it travels, this material pushes out clumps and streamers of gas into the galaxy’s expansive halo, where it eventually rains back in on the black hole, triggering the entire process anew.
In total, about three million solar masses of molecular gas is part of this fountain, forming a filamentary nebula that spans the innermost 100,000 light-years of the galaxy.
In an earlier study by the same authors published in the journal Nature, the researchers were able to verify the connection between the black hole and the galactic fountain by observing the region across a range of wavelengths, or portions of the electromagnetic spectrum. By studying the location and motion of molecules of carbon monoxide (CO) with ALMA, which shine brightly in millimeter-wavelength light, the researchers could measure the motion of the gas as it falls in toward the black hole.
Earlier data from the Multi-Unit Spectroscopic Explorer (MUSE) on ESO’s Very Large Telescope (VLT) revealed warm, ionized gas being expelled from the galaxy – essentially the plume of the fountain. The new ALMA observations found clumps of cold, molecular gas in precisely the same locations as the warm gas seen in the earlier observations.
“The unique aspect here is a very detailed coupled analysis of the source using data from ALMA and MUSE. The two facilities make for an incredibly powerful combination,” said Dr Tremblay. “ALMA revealed the distribution and motions of the cold molecular gas clouds, and MUSE did the same for the warm ionised gas.”
The observations also convincingly support the hypothesis that the warm ionised and cold molecular nebulas are one-and-the-same, with the warm ionised gas merely being the “shell” around the cold molecular cores that churn within this galaxy-scale fountain.
This multi-wavelength approach offers an uncommonly complete picture of this system. “It’s like observing the rain cloud, rain, and puddle all at the same time,” noted Dr Tremblay. While this is just one observation of one galaxy, the astronomers speculate that they may be observing a process that is common in galaxies and fundamental to their evolution.