by David Garofalo
We have been hearing about the unknown dark sector of the universe for decades, especially given recent tensions with the data. But black holes are responsible for the way galaxies form and evolve, the extent to which is likely underestimated. One feature of black holes that can affect their galactic and even intergalactic environment, has to do with their powerful jets. Jets coming from the centers of galaxies were discovered in 1917 from the galaxy whose black hole was recently imaged, M87. Nothing came of this until more evidence emerged. In the 1950’s an Armenian astronomer proposed an explanation for the energetic activity coming from the centers of some galaxies. He is responsible for the idea of an active galaxy but is mostly forgotten because he dismissed black holes as the explanation.
Today we have a coherent picture of what an active galaxy is. When lots of gas falls into black holes, they shine brightly. The biggest of these objects are called quasars and they were abundant when the universe was an adolescent, about 8 billion years ago. Roughly 20% of quasars also produce powerful beams of energy known as jets, called radio quasars, because we observe them at radio frequencies. Confidence is high that at these earlier times, galaxies collided more often with one another and this triggered both star formation as well as funneled gas deep into the gravitational potential where it ignited the merged black holes of the newly formed galaxy. Because the rate of galaxy collisions decreased over time, quasar numbers dwindled.
When the universe became a responsible adult it mellowed down a bit, and triggering black holes became more difficult. But even without a violent merger, some gas in galaxies can lose the angular momentum that keeps it away from galactic centers via a kind of friction, called secular processes, and fall toward the galactic center where the black hole awaits to feed. The galaxy becomes active and shines brightly but not at the level of a quasar. Within this class of active galaxy, astronomers are finding that jets are even less common. These non-merger triggered active galaxies tend to be flat or disk-like as a result of basic physical principles operating in isolated systems over long periods of time. When these bright, active disk galaxies also produce jets, they tend not to be as powerful as those from radio quasars. For a decade now astronomers took refuge in a comfortable idea which is that disk-like galaxies have less massive black holes at their centers, which would explain their weaker jet powers. But astronomers have recently found these jetted disk-like active galaxies to have black holes that overlap in terms of black hole mass with those in quasars, yet their jet powers are lower. Why? The nature of jet formation from black holes seems more puzzling than ever. But the solution is actually straightforward.
At the current epoch, it is possible to find jets from extremely massive black holes, but this requires observing in special environments, galaxy clusters, where the density of galaxies is large and the merger rate likely high in the past. However, one fails to find radio quasars here. Instead, we see radio galaxies, which are dull. If they did not have powerful jets, radio galaxies would hardly be noticed as active. How and why are jets at late time triggered in these dense environments and what makes them dull? Signs of mergers are not seen among these objects so perhaps they were triggered by secular processes such as in disk-like galaxies. But then, again, why are they so dull, unlike their disk-like active galaxy counterparts?
Decades ago, astronomers postulated that jets in active galaxies emerge somehow only from the most massive black holes. The chance that such an idea would work was not large even back then. Today, we know it fails. Perhaps, then, we can look to learn from jets observed in black holes at the smallest scales. Black hole X-ray binaries have black hole masses measured to be about a few tens of times the mass of the sun. These stellar-mass black holes live in binary systems with their companion star providing the gas that falls onto the black hole. These black holes sometimes shine brightly as miniature versions of quasars, sometimes are dull like miniature versions of radio galaxies, sometimes they have jets and oftentimes do not. And these different states appear in cycles that repeat over and over again. A long held hope is that active galaxies may be going through the same cycles that black hole X-ray binaries experience. But there’s a problem. If active galaxies behave this way, the distribution in time of active galaxies that reflects the different states of X-ray binaries, would be random. Instead, bright active galaxies with jets prefer to live in the past; those that are bright but jetless live in both disk-like galaxies as well as in galaxies that result from mergers; and those that are dull with jets prefer the present. The evolutionary sequence of X-ray binary black hole states is not, therefore, giving direct insight into the engine in active galaxies despite the fact that they are likely related.
So what is the nature of jets from black holes? If we date it from the discovery of the jet from the radio galaxy M87 in 1917, this remains the longest basic unanswered question in astrophysics. But the solution is actually right there in front of us.
Let’s give it a go. The crucial parameter for jet formation from black holes cannot be black hole mass since jets from black holes are observed across the mass scale. The crucial parameter for jet formation cannot be whether the material falling into the black hole shines brightly or is dull because jets have been observed from both black holes that are bright as well as dull. The crucial parameter cannot be the way the black hole is fed, via mergers, secular processes, or stellar companion, because jets have been observed from black holes fed in all those ways. So what is the crucial parameter?
One important element yet to be discussed is black hole spin. Black holes rotate and therefore carry rotational energy they can release to the universe. Models connecting black hole spin to jets have existed for over a half century. The idea is that jets are produced by black holes whose spin is large. If the spin is too low or zero, no jet is produced but compatibility with X-ray binaries suggests that a mechanism must exist that suppresses the jets at various times for black holes whose spin is large. Can we use black hole spin to explain the observations? Unfortunately things still fail to work out. Here’s the issue. And it’s a crucial one.
When gas falls into a black hole it gives the black hole its angular momentum and therefore spins it up. High black hole spin is the end product of gas falling into black holes regardless of the initial spin. Jets, as described, require high spin. Because high spin will on average occur later, jets should become dominant later rather than sooner. But the observations show the opposite: Radio quasars lived in the past while the peak of bright, jetless active galaxies occurs billions of years later. A model that predicts jets and brightness to go hand-in-hand cannot, therefore, explain the observations. The mental gymnastics that astro theorists have been practicing for decades is a subject for another time. Unfortunately the issue is one of principle but it has not yet been absorbed into our collective consciousness.
While black hole spin, disk brightness, environment, and black hole mass all matter, no clear picture emerges without the help of one additional parameter. That additional parameter has been the subject of my research over the past decade and is captured in the cartoon below. Welcome to my world.