How Did Black Holes Grow So Speedily? The Jets

Within almost every galaxy has a SMBH (supermassive black hole). The beast at the heart of our galaxy Milky Way contains the mass equivalent to millions of suns, while some of the largest SMBHs can be over a billion solar masses. For quite a long time, it was believed that these black holes grew in mass over time, only reaching their present size after a billion years or more. But observations from the Webb telescope (JWST) show that even the youngest galaxies contain colossal black holes. So how could SMBHs grow so large so quickly? The key to the answer could in fact be the powerful jets black holes can produce.

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

We can see quite a few fast-growing black holes in the faraway Universe as quasars, or active galactic nuclei. We know, then, that in the middle age of the universe, many SMBHs were gaining mass quickly. One idea is that the youngest SMBHs also had active jets, which would let them to gain a million solar masses or more very quickly. But proving this is hard.

The problem is that it’s very difficult to observe jets from the earliest period of the universe. Light from that distant time is so much redshifted that their once vivid beacon has become dim radio light. In a new study, the research team discovered a blazar with a redshift of z = 7.0, meaning it comes from a time when the Cosmos was just 750 million years old. A blazar occurs when the jet of a SMBH is lined up to be pointed directly at us. As we’re looking directly into the beam, we watch the jet at its most powerful.

Blazars normally let us to calculate the true intensity of a jet, but in this particular case, the redshift is so very strong that our conclusions must be a bit more subtle. One likelihood is that the jet of SMBH of this particular blazar is really pointed directly our way. The black hole of the blazar is growing so quickly that it would easily gain over a million solar masses within the first billion years of time. But it would be very rare for a black hole jet to point directly at us from that far distance. So statistically, that would basically mean there are many more early black holes that are just as active and growing just as fast. They simply aren’t aligned for us to observe.

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

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