Supermassive black holes sit at the center of almost all galaxies. Despite popular imagination, they don’t act as gigantic plug holes slowly "eating" everything that comes their way. Instead, they mostly sit idle. But if material does end up too close to them – such as during a galaxy collision – they can become active, releasing a large amount of radiation and powerful jets.
In the aftermath of the sudden awakening of a supermassive black hole, galaxy-wide winds are generated. Now, researchers have learned more about how these winds form.
As reported in Astronomy & Astrophysics, the scientists used eight years’ worth of data from the XMM-Newton X-ray observatory to study the ejection of material. Prior to this, astronomers were aware of two types of outflows: the first, called warm absorbers, is slow-moving, far away from the black hole, and similar in density to the interstellar medium. The second type is quick (up to 40 percent the speed of light) and made of highly ionized gas concentrated near the black hole. These are known as ultra-fast outflows, or (without a hint of irony) UFOs.
In the new work, the team discovered the existence of a third type of flow that's an in-between state. It has the physical characteristics of the first and the impressive speed of the UFOs. These phenomena happen between tens to hundreds of light-years from the black hole.
"We believe that this is the point when the UFO touches the interstellar matter and sweeps it away like a snowplough. We call this an 'entrained ultra-fast outflow' because the UFO at this stage is penetrating the interstellar matter. It's similar to wind pushing boats in the sea," lead author Dr Roberto Serafinelli of the National Institute of Astrophysics in Milan, said in a statement.
Models had previously predicted the existence of entrained UFOs (E-UFOs), but this is the first observational evidence of their existence. The UFOs propagate for years before reaching the interstellar medium, turning into E-UFOs and eventually into warm absorbers. They then continue to spread through interstellar space, becoming the winds observed in many galaxies.
"These winds might explain some surprising correlations that scientists have known about for years but couldn't explain," added Dr Serafinelli. "For example, we see a correlation between the masses of supermassive black holes and the velocity dispersion of stars in the inner parts of their host galaxies. But there is no way this could be due to the gravitational effect of the black hole. Our study for the first time shows how these black hole winds impact the galaxy on a larger scale, possibly providing the missing link."
XMM-Newton is a European Space Agency mission. Its successor, Athena (Advanced Telescope for High ENergy Astrophysics), will provide even more detailed insights into the effects of supermassive black holes on galaxies.