If the primary black holes collapse straight away, might we detect radio indicators from these moments?

The universe is littered with supermassive black holes. Just 30,000 light years away is one in the center of the Milky Way. Most galaxies have one, and some of them are more massive than a billion stars. We know that many supermassive black holes formed early in the universe. For example, the quasar TON 618 is powered by a black hole with a mass of 66 billion solar cells. Since its light travels almost 11 billion years to reach us, TON 618 was already huge when the universe was only a few billion years old. So how did these black holes become so massive so quickly?

One idea is that some of the very first stars were giants. With a mass of more than 10,000 suns, such a star would be very short-lived and would quickly collapse into a large black hole. These first black holes would act as seeds in the center of a galaxy, consuming nearby material to grow in size quickly. Some of them would even collide and merge into an even larger black hole. While it is a reasonable model, computer simulations determine that this process takes too long. This process cannot create the kind of black holes we see in the early universe like TON 618.

A direct image of the supermassive black hole in M87. Photo credit: EHT collaboration

Another idea is known as the direct collapse scenario. In this model, a small, supermassive black hole suddenly forms. Dense gas in the middle of a protogalaxy cools enough to collapse under its own weight and form a black hole. Because these black holes have a mass protrusion, they can quickly grow into the supermassive black holes that we observe.

So far we have not been able to observe a black hole with direct collapse (DCBH). A few years ago, some DCBH candidates were discovered based on their infrared signals. These could be confirmed when the James Webb Space Telescope (possibly) launches later this year. But recently a study argues that we could observe DCBHs through their radio signatures.

When black holes are actively consuming nearby matter, they can create powerful jets of hot plasma. These jets are radio loud and one of the ways we can identify supermassive black holes. Direct collapse black holes should have similar jets, but the jet material would be more dense. And since DCBHs would form in the early universe, their radio signals would be more redshifted. This latest work argues that the radio signature of DCBHs would have a similar structure but would be easily distinguishable from the radio jets we see today. The signature would also be different from jets created by black holes.

Unfortunately, these high-redshifted radio sources cannot be seen by current radio telescopes. However, they should be bright enough to be recognized by the Square Kilometer Array (SKA) and the proposed next generation Very Large Array (ngVLA).

Relation: Yue, B. and A. Ferrara. “Radio signals from early black holes that are directly collapsing.” Royal Astronomical Society monthly bulletin 506.4 (2021): 5606–5618.

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