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Solving the black hole-galaxy connection

by David Garofalo

One of the most fundamental correlations in astrophysics has just been understood. In every galaxy where measurements of the mass of the central supermassive black hole are possible, it is found that the random velocities of stars too far from the black hole to feel its gravity, are correlated with the mass of the black hole. Black holes and stars seem to know about each other. But if these stars cannot feel the black hole's pull, how does the correlation come to be? This has been an open question for the past generation.

In 2009 and with the help of experts on the observations of jets produced by supermassive black holes, I put together an explanation for how black holes generate energy over time and across the universe. The key to the formation of powerful jets from black holes is counter-rotation between the gas that rotates around and falls into the black hole, and the black hole. While many different observations appeared to fit naturally in this framework, one key fact appeared unexplained: Active galaxies whose supermassive black holes produce powerful relativistic jets of energy (so-called radio quasars), have many stars in their galactic center where the supermassive black hole resides. But active galaxies whose supermassive black holes do not produce jets, do not have many stars in their galactic centers. When I was putting the ideas together, this puzzled me. My instinct led me to believe that strong jets could evacuate the central region of stars, gas, and dust, but this was precisely opposite of what was observed. So what did I do? Like the researchers I criticize today for avoiding to deal with all the observations, I simply ignored the problem. But I knew it was something I would ultimately have to explain if this model were truly fundamental to the way black holes work. And I did.

Within a few years, the community experienced breakthroughs on how two black holes merge together in galaxy collisions, but we are getting ahead of ourselves. Already back in 2003, astrophysicists Milos Milosavljevic and David Merritt in the USA, showed that if the gas that surrounds the merging black holes rotates in the same direction as the binary, the black holes will be unable to extract enough angular momentum to bring them together and will thus remain rotating around each other at a characteristic distance of about 1 parsec. Therefore, to push the black holes further toward each other, they will need to give their excess angular momentum to stars in their vicinity, which pushes the stars away and evacuates the core region of the newly formed galaxy. The black holes can finally merge into one more massive black hole, and the remaining angular momentum will show up in the rotation of the black hole, which will likely be in the same direction as that of the flow of gas around it. A newly formed spinning black hole surrounded by a co-rotating disk of gas (called an accretion disk) is formed.

Around 2011, simulations of merging black holes by Chris J. Nixon in the UK showed that if the gas surrounding the two rotating black holes is in the opposite direction, the excess angular momentum of the binary is more easily extracted and no additional help from stars is required. Such a configuration can lead to the merged black hole rotating in the opposite direction of the gas surrounding it. Therefore, black holes that counter-rotate with respect to their accretion disks will not experience cores that are evacuated of stars.

In short, the evacuated/non-evacuated stellar cores in galaxies has nothing to do with jets, and all to do with the merging history of black holes. Recent work by Nandini Sahu, Alister W. Graham, and Benjamin L. Davis in 2019 in Australia, showed that on close inspection, the tight correlation between black hole mass and stellar velocity dispersion (the M-σ relation) is more subtle. For galaxies whose cores are evacuated of stars, the black hole mass grows more rapidly for a given σ value. For galaxies whose cores are not evacuated of stars, instead, the black hole mass grows less rapidly for a given σ value. In other words, there are two M-σ relations. This is a fundamental connection between black holes and galaxies that requires explanation.

Together with astrophysicist Damian J. Christian of the California State University, and others, we were very recently capable of producing quantitative estimates of the paths of jetted and non-jetted active galaxies in the M-σ diagram. And we found that active galaxies without jets have a steeper slope than those with jets. In other words, we found something that parallels the work of Sahu, Graham, and Davis but whose origin is completely different. There must be a connection.

The fundamental question therefore is this: why are/were galaxies without stars in their central regions active galaxies without jets and why are/were galaxies full of stars in their central regions active galaxies with jets? The original puzzle I faced back in 2009 is finally resolved in the following way. Counter-rotation is difficult to form, but when it does occur in galaxy mergers, it allows for the coalescing black holes to merge without appealing to stars to extract the excess angular momentum, and it constitutes an ideal configuration for powerful jet production. Because post mergers tend to produce co-rotating accretion disks, stars will be evacuated from the central region in order to merge the black holes and this will likely lead to a newly formed rotating black hole surrounded by a co-rotating accretion disk. This will not lead to powerful jet production.

The way things have come together over the last decade and a half teaches us two important lessons. The first is that counter-rotation between accretion disks and black holes is key to the formation of powerful jets from black holes. The second is that details about the black hole engine such as the spin of the black hole and the direction of gas rotation, has effects that are visible on galactic scales and beyond. If astronomers truly wish to understand galaxy formation and evolution, they will have to understand more about the details of what black holes do.

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