韦布望远镜拍到的小红点：是黑洞，还是黑洞星？

In the deep-space images sent back by the James Webb Space Telescope, more than five hundred small, red points of light are scattered.

Astronomers call them "little red dots," and to this day they cannot say exactly what these points of light are.

They are absurdly small—typically less than 0.5% of the Milky Way's width—yet their color is strikingly red.

The earliest ones were born 13.4 billion years ago, when the universe was only about 500 million years old.

When first discovered, astronomers thought they were distant objects packed with mature galaxies; some simply called them "universe breakers."

The trouble is, the universe was only 500 million years old—those galaxies simply wouldn't have had enough time to grow into that form.

In early 2023, Dale Kocevski of Colby College and his colleagues identified a recurring pattern in the spectra of several red dots.

This pattern can only come from a dense, rapidly moving cloud of hydrogen gas, and to maintain such speeds, an enormous mass must be packed into a tiny volume.

Packing so much mass into such a small space—the most natural explanation is a black hole.

Kocevski's camp therefore believes the red dots are ordinary accreting black holes, that is, active galactic nuclei, just wrapped in something different.

In the classic picture, an active galactic nucleus is surrounded by a doughnut of dust; in the red dots, it is replaced by layers of gas that redden the light along the way.

The difficulty is that such black holes should radiate copious X-rays, yet almost no trace of X-rays is detected from the red dots.

The same camp's explanation is that matter falls in so fast that it seals the X-rays inside.

Rohan Naidu of the University of Hawaii and his colleagues offer another scheme, which they name "black-hole stars."

Imagine a black hole wrapped in a shell of gas far denser than ordinary gas, with the outermost layer glowing red and bright like the surface of a star.

It looks like a star, but inside is a black hole.

Critics point out that in a few of the dots, light clearly leaks out from the gas shell—something the black-hole star model cannot account for.

Criticism of the black hole model is equally sharp: to justify itself, it requires a series of independent assumptions.

Kocevski himself says he has rarely seen a debate this fierce.

In May of this year, Ignas Juodžbalis, then at the University of Cambridge, led the publication of the first mass measurement for a red dot.

The measured mass is about 50 million solar masses; the black hole camp regards it as strong support.

Jenny Greene of Princeton University questions precisely this measurement method.

She points out that this method applies to systems with clear boundaries, such as a pair of stars orbiting each other, not to the motions within a messy blob of gas.

In her words, people often see what they want to see.

Juodžbalis responds that this is just one data point.

The two models' mass estimates differ by several orders of magnitude: black-hole stars are much lighter, while AGN-like objects can be much heavier.

Naidu's attitude is direct: "We just have to concede that this is a fundamentally new kind of object."

As long as the identity of the red dots remains unsettled, there is no answer to how the first supermassive black holes grew so large so quickly.

How they in turn shaped the surrounding galaxies also remains hanging in the balance.