Astronomers Peer Through Stellar Dust to See Black Holes Eating Whole Stars
A dormant black hole isn't really inactive; it's just all alone. Without any meaningful amount of material around to pull in, a black hole will simply fly through space and present virtually no signal for astronomers looking to find it. Though its mass will bend light in the same characteristic ways as any other black hole, astronomers generally need to see its effect on surrounding matter to identify candidate regions of the sky worth studying at all.
An artist's impression of a tidal disruption event. Credit: ESO/L. Calçada
However, these 'dormant' singularities will occasionally come into contact with stars and other objects as they move around the galaxy. If they come too close, these objects are gobbled up by the black holes. When the object is a large star, it can be ripped physically apart by the gravitational forces.
This star-destroying process is what's known as a tidal disruption event, and it's associated with a huge emission of radiation whenever matter is pulled over the event horizon. Usually, some proportion of the matter is converted to energy and blasted out in a form that astronomers can see.
The problem is that very dusty galaxies can hide these events, blocking the X-ray or visible light emissions and hiding information that could help understand black holes and their galaxies. These researchers had previously supposed that in such cases, the radiation from the tidal disruption event should interact with the dust blocking it, producing infrared light that could be detected instead.
The James Webb Space Telescope. Credit: NASA
So they turned to the Webb telescope, the most advanced infrared detector in existence, to look for these characteristic signals—and found them.
The issue was that these galaxies didn't appear to have the structure of an active black hole with a permanent accretion disc. Instead, they looked to be dormant black holes that were transiently eating up a star, producing a short burst of radiation, and then going back to quiescence.
Tidal disruption events are surprisingly rare to see, with only a few dozen having been seen in total, but many scientists believe they are actually more common, and just often hidden from view.
This study shows how even occluded black holes could be studied in the future—and that even hard-to-study singularities will often provide useful inroads for experimentation.
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