How Did Black Holes Grow So Quickly? The Jets

Within nearly every galaxy is a supermassive black hole. The beast at the heart of our galaxy contains the mass of millions of suns, while some of the largest supermassive black holes can be more than a billion solar masses. For years, it was thought that these black holes grew in mass over time, only reaching their current size after a billion years or more. But observations from the Webb telescope show that even the youngest galaxies contain massive black holes. So how could supermassive black holes grow so large so quickly? The key to the answer could be the powerful jets black holes can produce.

Although it seems counterintuitive, it is difficult for a black hole to consume matter and grow. The gravitational pull of a black hole is immensely strong, but the surrounding matter is much more likely to be trapped in orbit around the gravitational well than to fall directly in. To enter a black hole, material needs to slow down enough to fall inward. When a black hole has a jet of material speeding away from its polar region, this high-velocity plasma can pull rotational motion from the surrounding material, thus allowing it to fall into the black hole. For this reason, black holes with powerful jets also undergo the most powerful growth.

We can see many fast-growing black holes in the distant Universe as quasars, or active galactic nuclei. We know, then, that in the middle age of the cosmos, many supermassive black holes were gaining mass rapidly. One idea is that the youngest supermassive black holes also had active jets, which would allow them to gain a million solar masses or more quite quickly. But proving this is difficult.

The problem is that it’s extremely difficult to observe jets from the earliest period of the cosmos. Light from that distant time is so redshifted that their once brilliant beacon has become dim radio light. Before this recent study, the most distant jet we observed had a redshift of z = 6.1, meaning it traveled for nearly 12.8 billion years to reach us. In this new study, the team discovered a blazar with a redshift of z = 7.0, meaning it comes from a time when the Universe was just 750 million years old.

A blazar occurs when the jet of a supermassive black hole is lined up to be pointed directly at us. Since we’re looking directly into the beam, we see the jet at its most powerful. Blazars normally allow us to calculate the true intensity of a jet, but in this case, the redshift is so strong that our conclusions must be a bit more subtle.

One possibility is that the jet of this particular supermassive black hole really is pointed directly our way. Based on this, the black hole is growing so quickly that it would easily gain more than a million solar masses within the first billion years of time. But it would be extremely rare for a black hole jet to point directly at us from that distance. So statistically, that would mean there are many more early black holes that are just as active and growing just as quickly. They just aren’t aligned for us to observe.

Another possibility is that the blazar isn’t quite aligned in our direction, but the cosmic expansion of space and time has focused its energy toward us over 12.9 billion years. In other words, the blazar may appear more energetic than it actually is, thanks to relativistic cosmology. But if that is the case, then the jet of this black hole is less energetic but still powerful. And statistically, that would mean most early black holes are equally powerful.

So this latest work tells us that either there was a fraction of early black holes that grew to beasts incredibly fast, or that most black holes grew quickly, beginning at a time even earlier than we can observe. In either case, it is clear that early black holes created jets, and these jets allowed the first supermassive black holes to appear early in cosmic time.

Reference: Bañados, Eduardo, et al. “A blazar in the epoch of reionization.” Nature Astronomy (2024): 1-9.

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