Growing Black Holes Have Much in Common With Baby Stars

First looks would tell most observers that supermassive black holes (SMBHs) and very young stars have nothing in common. But that’s not true. Astronomers have detected a supermassive black hole (SMBH) whose growth is regulated the same way a baby star’s is: by magnetic winds.

Supermassive Black Holes are so massive that comprehending them is difficult. They can be billions of times more massive than our Sun, a number so easy to say that it trivializes their true magnitude. They grow so large through two mechanisms: mergers and accretion.

Black holes can’t be seen directly, but their existence is confirmed by observing how they alter their surroundings. SMBHs are so massive that they alter the orbits and velocities of nearby stars, a phenomenon astronomers have clearly observed. SMBHs are also visible as active galactic nuclei when they’re actively accreting material. Lastly, when black holes merge, they release gravitational waves that we can detect with facilities like LIGO/Virgo.

But there are lots of unanswered questions about how black holes grow by accretion. To try to understand how an SMBH accretes gas and acquires mass, a team of researchers observed ESO320-G030, a nearby galaxy only 120 million light years away.

Their results are in a paper titled “A spectacular galactic scale magnetohydrodynamic powered wind in ESO 320-G030.” The paper is published in the journal Astronomy and Astrophysics, and the lead author is Mark Gorski, a postdoc at Northwestern University.

One outstanding issue in the study of SMBHs concerns black hole feedback. Not all of the material that enters an SMBH’s accretion disk falls into the hole. Some is released by astrophysical jets. This is part of a process called black hole feedback, and it shapes how the black hole grows and how quickly its galaxy forms new stars.

ESO 320-G030 is interesting not only because it hosts an SMBH but also because it’s forming new stars at a rapid rate, about ten times as fast as the Milky Way. To try to understand all the processes in the galaxy’s nucleus, a team of researchers used the Atacama Large Millimetre/submillimetre Array (ALMA) to observe molecules being transported from the galaxy’s center outward.

“How galaxies regulate nuclear growth through gas accretion by supermassive black holes (SMBHs) is one of the most fundamental questions in galaxy evolution,” the authors write in their research article. “One potential way to regulate nuclear growth is through a galactic wind that removes gas from the nucleus.”

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

“It is unclear whether galactic winds are powered by jets, mechanical winds, radiation, or via magnetohydrodynamic (MHD) processes,” the authors write. By using ALMA to observe HCN, the researchers hoped to bring clarity.

ESO 320-G030 is a particular type of galaxy. It’s a luminous infrared galaxy with a very compact nucleus obscured by dust. About 30% of these types of galaxies have extremely compact nuclei with growing SMBHs or unusual starbursts. There’s clearly a lot of action in the galaxy’s nucleus, so it’s a critical target for astrophysicists and astronomers.

“Since this galaxy is very luminous in the infrared, telescopes can resolve striking details in its centre,” said Susanne Aalto, Professor of Radio Astronomy at Chalmers University of Technology. “We wanted to measure light from molecules carried by winds from the galaxy’s core, hoping to trace how the winds are launched by a growing, or soon to be growing, supermassive black hole. By using ALMA, we were able to study light from behind thick layers of dust and gas.”

There’s a debate among astronomers over the nature of black hole feedback. Galaxies have AGN-driven outflows that inject gas back into a galaxy’s nucleus, but they can’t agree on the nature of the feedback. It could be jets, mechanical winds, or radiation. Observing ESO 320-G030 with ALMA’s molecule-observing ability is a chance to wade deeply into the debate.

ALMA was able to trace the behaviour of HCN due to excitational vibration. The observations result in maps of the molecule’s movement in the galaxy’s nucleus.

“We can see how the winds form a spiralling structure, billowing out from the galaxy’s centre. When we measured the rotation, mass, and velocity of the material flowing outwards, we were surprised to find that we could rule out many explanations for the power of the wind, star formation for example. Instead, the flow outwards may be powered by the inflow of gas and seems to be held together by magnetic fields,” said Aalto.

As the SMBH draws material into its rotating accretion disk, the rotation creates powerful magnetic fields. The magnetic fields lift matter away from the center, creating a spiralling MHD (magnetohydrodynamic) wind. As matter is removed by the wind, the disk rotation slows. Slower rotation allows more material to fall into the hole, letting the SMBH grow more massive.

Other winds and jets in the nucleus propel material away from black holes in galaxy nuclei, but this newly discovered wind feeds material into the black hole. “In this Letter, we present compelling evidence that the outflow in ESO 320-G030 is powered by a different mechanism, an MHD wind launched prior to the ignition of an AGN,” the authors write. Since an AGN is observed when an SMBH has accreted material into its disk and the material has been heated by rotation, the wind the researchers observed is likely responsible for feeding material into the black hole’s disk, some of which falls into the hole itself.

To the astronomers behind the work, the ALMA data images are a breathtaking new insight into the winds in ESO 320-G030’s galactic nucleus. “What is spectacular about the outflow morphology is that the launching regions are apparent and connected to the rotating nuclear structure in the innermost ~12 pc,” they write. The patterns revealed by ALMA hint at the presence of a magnetized rotating wind.

The wind’s rotating element is key. “The rotation of outflows is a strong indication of magnetic acceleration,” the authors explain. If magnetic acceleration is driving it, then the other phenomena astronomers debate—AGN, astrophysical jets, or radiation—can’t be responsible.

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

“It is well-established that stars, in the first stages of their evolution, grow with the help of rotating winds – accelerated by magnetic fields, just like the wind in this galaxy. Our observations show that supermassive black holes and tiny stars can grow by similar processes, but on very different scales.” said lead author Gorski in a press release.

This could be a big step in understanding how SMBHs grow, but the authors know it’s only one step. They need to observe more SMBHs and gather more data before anything is conclusive.

“Far from all questions about this process are answered. In our observations we see 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 find out how common a phenomenon this is. And if this is a stage which all galaxies with supermassive black holes go through, what happens to them next?” asks lead author Gorski.