For the first time, the James Webb Space Telescope (JWST) has glimpsed the hidden exploits of dormant black holes. In clearer galaxies, these are typically observed by tidal disruption events (TDEs): cases where a galaxy’s central supermassive black hole gradually pulls in and tears a nearby star apart, releasing an intense surge of energy. Dusty galaxies; however, obscure x-ray and optical radiation emitted by TDEs so that these sleepy black holes are usually impossible to see. Published on July 24 in The Astrophysical Journal Letters, this study elaborates on how astronomers are instead using the JWST to detect infrared light, allowing them to investigate the unknown environment of a dormant black hole.
Unlike X-ray and optical radiation, infrared light is not as easily absorbed by galaxy dust. Focusing on signals of infrared light, astrophysicist Megan Masterson and her team at MIT returned to data from NASA’s NEOWISE mission on twelve different signals thought to be produced by TDEs. Of the 12 signals, they identified four to be marked with a telltale sign of a TDE: infrared emissions from atoms that had electrons stripped by the black hole’s radiation. JWST’s detection of these spectral lines affirmed that in those four galaxies, black holes had devoured stars.
Exploring further, researchers wanted to confirm the nature of these black holes. Is accretion a brief feature initiated by a TDE from a black hole that just happened to become active as a star passed? Or, is accretion a long-term feature of an always active black hole? Studying those galaxies, the team made two key observations.
First, there were clear signs of black hole accretion. This is the process by which black holes eventually consume material like stellar debris.
Second, researchers discovered dust patterns deviating from those found in active galaxies. They utilized the JWST to gather data of another infrared light wavelength capable of showing the presence of silicates. They were able to map patterns of the galaxy dust in each of the four galaxies, which didn’t resemble the patterns of active galaxies containing clumpy “donut-shaped” clouds.
This further implies that the black holes of these dusty galaxies are not usually active, but truly dormant up until an unfortunate star approached and was subsequently destroyed. An accretion disk around those black holes, then, would have been formed by a TDE.
Use of the JWST’s ability to observe infrared light sets a remarkable precedent for the study on black holes. By multitudes of TDE detections, TDEs have potential to be incredibly effective probes for black hole properties.
“The actual process of a black hole gobbling down all that stellar material takes a long time,” Masterson expressed. “It’s not an instantaneous process. And hopefully, we can start to probe how long that process takes and what that environment looks like. No one knows because we just started discovering and studying these events.”
