🔭 James Webb discovers unexpected 'naked' black hole

A black hole that weighs more than the rest of the galaxy hosting it. Discovered using the James Webb telescope in the primordial universe, this object named QSO1 appears to have formed even before its host galaxy had time to take shape.

The 'little red dots' are enigmatic objects from the early universe. Until now, indirect estimates of the mass of the black holes they contain relied on local assumptions, which are hotly debated. But a new study published in Nature used a direct method: spectroastrometry, which measures the orbital motion of gas to weigh the central black hole.


The technique used exploits the fact that the gas rotates faster the closer it is to the black hole. By analyzing hydrogen lines with JWST's spectrograph, astronomers reconstructed the rotation curve. This made it possible to estimate the black hole's mass at about 50 million suns.

In parallel, the total mass of the stars in the host galaxy was assessed at less than 20 million solar masses. The black hole is therefore more than twice as heavy as its galaxy. This is the most massive 'naked' black hole ever observed, indicating that it grew before its galaxy formed.

This observation contradicts the standard scenario where black hole and galaxy grow together. The black hole in QSO1 could have formed through the direct collapse of a primordial gas cloud, or it could be a primordial black hole born just after the Big Bang. Current data does not allow a definitive conclusion.

Researchers plan to study other 'little red dots' to see if this phenomenon is widespread. This discovery offers a new window into the formation of the first supermassive black holes in the universe, and could explain how they reached such high masses so quickly.

Gravitational lensing

This phenomenon occurs when a massive object, such as a galaxy cluster, warps the spacetime around it. Light from a more distant source is then bent and magnified, as if passing through a magnifying glass. In the case of QSO1, the cluster Abell 2744 increased its brightness sixfold and stretched its image. This allowed astronomers to detect minute details that the telescope would not otherwise have been able to see. Gravitational lensing is a valuable natural tool for observing the most distant objects in the universe.

This technique makes it possible to probe regions otherwise inaccessible. By combining several distorted images, researchers can reconstruct the source's structure. In this study, lensing was essential for measuring the rotation of the gas around the black hole.

Without this phenomenon, JWST's resolution would not have been sufficient to distinguish the internal motions of QSO1.

Spectroastrometry

This method allows measuring very small shifts in the position of a light source at different wavelengths. When gas orbits a black hole, spectral lines are redshifted or blueshifted depending on whether the gas is moving away from or toward us. By analyzing these shifts in the image, one can map the gas velocity at different distances from the center.

Spectroastrometry pushes the limits of telescope resolution. Here it allowed reconstructing the rotation curve of the gas.

Only a model with a very massive black hole matched the observations. This technique therefore provided the first direct measurement of a black hole's mass in a 'little red dot'. It paves the way for other similar studies in the primordial universe.