Astronomers have used a technique called echo mapping to detect hints that supermassive black holes, such as the cosmic titan at the heart of the Milky Way, known as Sagittarius A* (Sgr A*), are surrounded by dense clouds and clusters of dark matter. The research could teach us more about this mysterious substance and the environments around supermassive black holes.
Dark matter is the universe’s most mysterious stuff, outweighing ordinary matter in the cosmos by a ratio of five to one — but remaining effectively invisible because it doesn’t interact with electromagnetic radiation, including the light we use to see. The only way scientists can even infer the presence of dark matter is via its interaction with gravity, and the impact that this interaction has on objects made of traditional matter like stars. For instance, the gravitational effect of dark matter allows stars at the edges of galaxies to whip around at much greater speeds while not flying loose than the visible matter of those galaxies would allow.
This team decided to test the gravitational influence of dark matter at the hearts of galaxies, environments dominated by supermassive black holes which can have masses millions or even billions of times that of the sun. Ordinary matter around these supermassive black holes is often very visible, especially when spiraling into the maw of one of these cosmic titans from a flattened cloud called an accretion disk. This is because the gravitational influence of those black holes generates immense amounts of friction, causing them to grow brightly. That wouldn’t work for dark matter; it can’t feel friction because it doesn’t interact with itself or with ordinary matter, and it can’t glow because it doesn’t absorb or emit light.
Clearly, dark matter can’t be spotted around supermassive black holes even using the most advanced telescopes such as the Event Horizon Telescope (EHT), which has captured glowing rings of material around Sgr A* and around a more distant supermassive black hole that rules the heart of the galaxy Messier 87 (M87).
While discussing the problem of detecting dark matter around supermassive black holes, Mayank Sharma, a physics graduate student at Virginia Polytechnic Institute and State University (Virginia Tech), hit on an interesting solution.
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“We could actually test this prediction using a technique in astronomy, which allows you to measure the distance to the surrounding gas by looking for echoes of light,” Sharma said in a statement. The technique Sharma refers to is “reverberation mapping,” and it has become a trusted method of determining the mass of black holes.
Reverberation mapping is based upon the fact that as matter falls into a black hole, it releases a burst of energy that causes the accretion disk it comes from to pulse. This pulse of light travels from the accretion disk to gas in the wider environment of the black hole. This gas absorbs that light and also pulses, with this secondary pulse serving as an echo of the first.
Because we know the speed of light, when astronomers see the first pulse of light and then its echo, they can use the time between pulses to estimate the distance between the black hole and the gas on the outskirts of its environment. The size of a black hole and the distance between it and outer gas clouds can be used to determine its mass, and could also be used to determine the mass of dark matter clustered around it.
The team applied their method to 14 different galaxies, finding in five cases that mass increases moving away from the central black hole in a way that couldn’t be accounted for by visible matter alone. Despite the early success of this research, it far from proves that supermassive black holes are indeed gathering places for dark matter. The team’s findings do point an interesting way forward for the investigation into the universe’s most mysterious substance and its most mysterious regions.
“These galaxies are definitely showing a hint that there is extra material that cannot be explained by just the supermassive black hole,” Sharma said. “The prospects are exciting.”
The team’s research was published in the journal Physical Review D.