A supermassive black hole is speeding through space, and astronomers don’t know why
Asupermassive black hole is racing across the universe at 177,000 kilometres (110,000 miles) per hour, and the astronomers who spotted it don’t know why. The fast-moving black hole, which is roughly 3 million times heavier than our Sun, is zipping through the centre of a galaxy about 230 million light years away.
Scientists have long theorised that black holes could move, but such movement is rare because their giant mass requires an equally enormous force to get them going. “We don’t expect the majority of supermassive black holes to be moving; they’re usually content to just sit around,” said Dominic Pesce, an astronomer at the Harvard-Smithsonian Center for Astrophysics.
To begin their search for this infrequent cosmic occurrence, researchers compared the velocities of ten supermassive black holes with the galaxies they formed the centre of, focusing on the black holes with water inside their accretion discs – the spiral-shaped collections of cosmic material in orbit around the black holes. Why water? As water orbits a black hole, it collides with other material, and the electrons surrounding the hydrogen and oxygen atoms that make up water molecules get excited to higher energy levels. When these electrons return to their ground state, they emit a beam of laser-like microwave radiation called a maser. By taking advantage of a cosmic phenomenon known as redshift, in which objects moving away have their light stretched to longer wavelengths, the astronomers were able to observe the extent to which the maser light from the accretion disc was shifted away from its known frequency when stationary, thereby gauging the speed of the moving black hole.
They took more observations from various telescopes and combined them all together using a technique called very-longbaseline interferometry. With this technique, the researchers could combine the images from several telescopes to effectively act like an image captured by a very big telescope, about the size of the distance between them. In that way the scientists could precisely measure the velocity of the black holes it originated from.
The researchers don’t know what could have made such a heavy object move at such a high speed, but they came up with two possibilities. “We may be observing the aftermath of two supermassive black holes merging,” said Jim Condon, a radio astronomer at the National Radio Astronomy Observatory, “The result of such a merger can cause the newborn black hole to recoil, and we may be watching it in the act of recoiling or as it settles down again.” The other possibility is considered to be much rarer and more novel: the supermassive black hole may be part of a pair with another black hole that’s invisible to measurements. If the black hole is being tugged around by an even bigger invisible one, this could explain why it’s travelling so fast, but more observations will be needed to get to the bottom of the mystery.