Astronomers weighed a ‘little red dot’ discovered by the James Webb telescope — and found a ‘naked’ black hole inside

Earlier indirect estimates had placed black hole QSO1’s mass at around 40 million solar masses ‪—‬ remarkably high for such a compact, young system. The larger and more isolated the black hole is relative to its surroundings, the bigger its sphere of influence ‪—‬ the region where its gravity dominates over the stars, gas and dark matter around it. A high-mass black hole, therefore, makes it easier to detect its gravitational influence in the motions of nearby gas.

Adding to this, the galaxy cluster Abell 2744 — located between us and QSO1 — is so massive that its gravity acts as a magnifying glass. Through this effect, known as gravitational lensing, astronomers can see QSO1 brightening by a factor of six and stretching it spatially by a factor of 3.5.

Get the world’s most fascinating discoveries delivered straight to your inbox.

The James Webb Space Telescope has detected dozens of peculiar ‘little red dots’ in the early universe. The new study hints that some of them may be ancient black holes that took shape even before galaxies formed around them.

(Image credit: Bangzheng “Tom” Sun)

Overall, these conditions were favorable to make the kind of measurement the researchers were aiming for.

The measurement technique relied on the principle that gas orbiting a black hole moves faster the closer it gets to it. By mapping how fast gas moves at different distances from the center of QSO1, the team could work backward to calculate the mass of whatever sat at the center.

The team used JWST’s Near-Infrared Spectrograph to extract maps of hydrogen emission line gas, tracking which parts of QSO1 were moving toward us and which were moving away. But the object was so small and distant that the rotation signal in the inner regions fell below what JWST could directly resolve.

What to read next

So they used spectroastrometry, a technique that measures tiny positional shifts in the light emitted by glowing gas across different wavelengths. This method can recover spatial information far below the telescope’s nominal resolution. “This way, we were able to reconstruct the rotation curve below the instrumental resolution of JWST,” Juodžbalis explained.

The team placed an upper limit on the mass of the stars in the host galaxy at around 20 million solar masses. This means the black hole significantly outweighs its entire host galaxy. Astronomers call such objects “naked” black holes, and QSO1 appears to be the most massive of this kind ever found.

With its massive black hole and its near-absent host galaxy, QSO1 appears to be a massive black hole seed caught in the very first stages of growth, before its galaxy had a chance to develop around it. This finding challenges the standard picture in which black holes grow together with their galaxy over billions of years.

The team considered two exotic origin scenarios for this black hole: direct collapse black holes, which form when massive clouds of pristine gas collapse straight into a black hole without forming stars first, and primordial black holes, which would have formed in the first second after the Big Bang.

“Both scenarios are exotic, and the current data and theory are not quite able to distinguish them,” Juodžbalis said.

The team plans to use upcoming ground-based observations to probe the black holes within similar objects that have been found in the local universe.