Growing black holes found to have much in common with newborn stars

Under the influence of magnetic fields, the spiral wind helps the supermassive black hole grow in the galaxy ESO320-G030. In this illustration, the galaxy's core is dominated by a rotating wind of dense gas escaping from the (hidden) supermassive black hole at the center of the galaxy. Gas movements, tracked by light from hydrogen cyanide molecules, were measured using the Atacama Large Millimeter/submillimeter Array.

At first glance, most observers might say that supermassive black holes (SMBHs) and very young stars have nothing in common. But that's not true. Astronomers have discovered a supermassive black hole, the growth of which is regulated in the same way as the growth of a baby star: by magnetic winds.

Supermassive black holes are so massive that they are difficult to imagine. They may be billions of times more massive than our Sun, a number so easy to pronounce that it detracts from their true magnitude. They become so large through two mechanisms: merger and accretion.

Black holes cannot be seen directly, but their existence is confirmed by observing how they change their environment. SMBHs are so massive that they change the orbits and velocities of nearby stars, a phenomenon that astronomers have clearly observed. SMBHs also act as active galactic nuclei when they are actively accreting material. Finally, when black holes merge, they emit gravitational waves that we can detect using instruments like LIGO/Virgo.

But there are many unanswered questions, particularly about how black holes grow through accretion. To try to understand how a SMBH accretes gas and gains mass, a team of researchers observed ESO320-G030, a nearby galaxy just 120 million light-years away.

Their results are presented in a paper entitled “Spectacular galactic-scale magnetohydrodynamic wind in ESO 320-G030” The work was published in the journal Astronomy and Astrophysics, and the lead author is Mark Gorski, a postdoc at Northwestern University.

One of the unsolved questions in the study of SMBHs concerns black hole feedback. Not all of the material that falls into the accretion disk of a SMBH ends up in the hole. Some of it is ejected as jets—relativistic streams. This is part of a process called black hole feedback, and it determines how the black hole grows and how quickly new stars form in its galaxy.

ESO 320-G030 is interesting not only because it hosts a SMBH, but also because it is forming new stars at a tremendous rate—about ten times faster than the Milky Way. To try to understand all the processes at the galactic core, a team of researchers used the Atacama Large Millimeter/submillimeter Array (ALMA) to observe molecules moving from the center of the galaxy outward.

“How galaxies regulate core growth through gas accretion from supermassive black holes (SMBHs) is one of the most fundamental questions in galaxy evolution,” the authors write in their paper. “One potential way to regulate core growth is through galactic winds that remove gas from the core.”

ALMA’s strength lies in its ability to see through thick gas and dust to observe light that lies between infrared and radio waves. It can track cold molecules by the light they emit in these wavelengths. In this study, ALMA tracked HCN (hydrogen cyanide) passing through the core of ESO 320-G030.

“It is not yet clear whether galactic winds are driven by jets, mechanical winds, radiation, or magnetohydrodynamic (MHD) processes,” the authors write. By using ALMA to observe HCN, the researchers hope to provide clarity.”

The jet of a supermassive black hole through the eyes of an artist. — An artist's impression of a supermassive black hole's jet.

ESO 320-G030 is a special type of galaxy. It is a galaxy glowing in the infrared with a very compact core obscured by dust. About 30% of galaxies of this type have very compact cores with growing supermassive black holes or unusual bursts of star formation. The galaxy’s core is clearly very active, making it a prime target for astrophysicists and astronomers.

“Because this galaxy is so luminous in the infrared, telescopes can resolve amazing details in its centre,” says Susanne Aalto, professor of radio astronomy at Chalmers University of Technology. “We wanted to measure the light coming from molecules carried by winds from the galaxy's core, in the hope of tracking how these winds are driven by a growing, or soon to be growing, supermassive black hole. With ALMA, we were able to study the light penetrating through thick layers of dust and gas.”

There is debate among astronomers about the nature of feedback between black holes. There are gas flows in galaxies driven by active galactic nuclei (AGNs) that force gas back into the galactic core, but they cannot agree on the nature of the feedback. These could be jets, mechanical winds or radiation. Observations of ESO 320-G030 using ALMA's molecular-tracking capabilities provide a deep dive into the debate.

ALMA was able to track the behavior of HCN due to its exciting oscillations. The observations resulted in maps of the molecule's motion in the galaxy's core.

This figure from the study shows the intensity-weighted velocity field of HCN in the core of ESO 320-G030. The authors write: "The approximate location and direction of outflow is shown by dotted arrows". The contours in the figure show that the emission of HCN vibrations "is extended along the outflow and that the outflow is launched from the equally rotating sides of the core". — This figure from the study shows the intensity-weighted velocity field of HCN in the core of ESO 320-G030. The authors write: “The approximate location and direction of outflow are indicated by dotted arrows.” The contours in the figure show that the HCN vibration emission is “stretched along the outflow and that the outflow is triggered from the equally rotating sides of the core.”

“We see winds forming a spiral structure emerging from the center of the galaxy. When we measured the rotation, mass and speed of the material flowing outwards, we were surprised to find that we could rule out many explanations for the strength of this wind, such as star formation. Instead, the outflow could be driven by an incoming flow of gas and appears to be held together by magnetic fields,” Aalto says.

As the SMBH pulls matter into its rotating accretion disk, the rotation creates powerful magnetic fields. Magnetic fields pull matter away from the center, creating a spiraling magnetohydrodynamic (MHD) wind. As the wind removes matter, the rotation of the disk slows down. Slowing down the rotation allows more matter to fall into the hole, allowing the SMBH to grow more massive.

Other winds and core jets push material away from black holes in the cores of galaxies, but this newly discovered wind feeds material into the black hole. “In this letter, we provide compelling evidence that the outflow in ESO 320-G030 is driven by a different mechanism, the MHD wind, initiated before the onset of the AGN,” the authors write. Because an AGN is observed when a SMBH accretes material into its disk, and this material is heated by the rotation, the wind observed by the researchers is likely responsible for feeding material into the black hole disk, some of which ends up in the hole itself.”

According to the astronomers behind the work, the ALMA images provide exciting new insights into the winds at the galactic core of ESO 320-G030. “What is impressive about the outflow morphology is that the firing regions are apparently associated with a rotating structure located in the interior of the core, within a radius of about 12 pc,” they write. The patterns identified by ALMA hint at the presence of magnetized rotating wind.

The main thing that gives rotation to matter is the wind. “The rotation of the outflows is a clear sign of magnetic acceleration,” the authors explain. If its driving force is magnetic acceleration, then other phenomena that astronomers argue about—AGN, astrophysical jets, or radiation—cannot be responsible.”

This newly discovered wind is similar to winds around young protostars that are accreting material and growing vigorously.

An artist's concept of a star being born in a protective shell of gas and dust. New research shows that magnetic winds drive the growth of both protostars and SMBHs. — An artist's concept of the birth of a star within a protective shell of gas and dust. A new study shows that magnetic winds drive the growth of both protostars and SMBHs.

“It is well known that stars in the early stages of their evolution grow with the help of rotating winds – accelerated by magnetic fields, like the wind in this galaxy. Our observations show that supermassive black holes and tiny stars can grow due to similar processes, but on completely different scales , says the paper's lead author, Gorski, in a press release.

This could be a big step in understanding how supermassive black holes grow, but the authors know it's only one step. They need to observe more SMBHs and collect more data before they can say anything definitive.

“There are still many unanswered questions about this process. Our observations show clear evidence of a rotating wind that helps regulate the growth of the galaxy's central black hole. Now that we know what to look for, the next step is to figure out how common this phenomenon is. And if this is a stage that all galaxies with supermassive black holes go through, what happens to them next?” asks lead author Gorski.