The Webb space telescope spies its first black holes snacking on stars

The James Webb Space Telescope has taken its first look at black holes secretly snacking on stars within dusty galaxies. JWST’s ability to pick up detailed infrared signals lets it peer past the dust to probe the mostly hidden black holes, researchers report in the Aug. 1 Astrophysical Journal Letters.

A dormant black hole is normally impossible to see. That changes when a star wanders too close. The black hole’s gravity stretches the star into a disk that rotates around and feeds the temporarily awakened giant in a tidal disruption event, or TDE. The disk of star gas heats up and emits X-rays and ultraviolet and visible light, which is how astronomers typically find sleepy black holes.

But “these wavelengths can be basically blocked” if dust shrouds a feasting black hole, says MIT astrophysicist Megan Masterson. Most known TDEs took place in fairly clear environments, but the events probably happen just as often in dusty, obscured galaxies, she and her colleagues recently reported. They’re just harder to see.

That dust, however, gives off its own signals. It releases infrared light after absorbing the gas’s emissions in other wavelengths. In previous work, Masterson and colleagues searched archived data from an infrared-based space survey and spotted 12 probable TDEs.

Masterson and colleagues set JWST’s sights on four of those TDEs. JWST, which sees in a wider range of infrared wavelengths than previous telescopes, detected infrared emissions from atoms that had been stripped of electrons through strong X-ray and ultraviolet radiation. That’s a telltale sign of a dining black hole.

Bright gas disks around fully awake black holes, which feed constantly and are surrounded by dust clumps, could give off similar radiation. But the observed signs of silicate dust looked more like specks swirling around a dormant black hole briefly waking up for a stellar snack, the team found. Computer simulations confirmed that TDEs could explain what JWST saw.  

Infrared light emissions are delayed by a few months compared with the typically detected wavelengths because it takes time for the shredded star’s light to reach the dust. But they’re essentially the only way to study feasting black holes blanketed by dust, Masterson says.