Finding Hidden Black Holes
March 27, 2025
Black holes are among the most mysterious objects in the universe. Nothing, not even light, can escape a black hole, making it completely invisible to telescopes. Looking for a black hole is like playing hide-and-seek in pitch darkness.
Yet astronomers have made a groundbreaking discovery: For the first time, an international team of astronomers detected hot gas, revealing a supermassive black hole that had been hiding in the early Universe for almost 13 billion years. How did astronomers find this massive monster?
Due to their immense gravity, black holes can gobble up matter nearby, resulting in rings of gas and dust moving around them. Some supermassive black holes can become quasars – some of the brightest objects in the cosmos.
The quasars eat way too much that all of the dust and gas falling into the black hole becomes super hot and begins to glow bright, emitting light across the electromagnetic spectrum. Most of the emitted light, for example visible light and X-ray, get blocked by thick layers of cosmic dust and can’t pass through the accretion disk around them. As a result, these monster black holes can remain hidden most of the time when astronomers observe the Universe using an X-ray telescope for example.
Here’s the twist. Unlike visible light, radio waves from hot gasses are not blocked that easily by cosmic dust and can be detected by telescopes on earth. “I know! I am very good at playing hide and seek with black holes”, quotes radio waves.
Thanks to the ultra high resolution observational capabilities of the Atacama Large Millimeter/submillimeter Array (ALMA) telescope, a team of astronomers detected radio light coming from hot and highly energetic gas (carbon monoxide) - a telltale sign that an active massive monster is lurking close by, thereby uncovering the supermassive black hole in hiding.
Astronomers are now thrilled to apply this technique to discover more supermassive black holes hiding in the early Universe. Cool right?
Image: Illustration inspired by ALMA data. When viewed from the side, visible light and X-rays are blocked by the disk, making the supermassive black hole at the center to remain hidden. Credit: ALMA (ESO/NAOJ/NRAO), K. Tadaki et al.