Turbulent environments surround black holes

An international team of researchers used a variety of telescopes to observe the outflow of gases from the supermassive black hole at the center of the Markarian 509 galaxy. Their findings suggest that these outflowing gases originate many light years away from the black hole.

In addition, they also found evidence that the light emitted from the fast-spinning gas near the black hole is energized as it passes through and interacts with much higher temperature gas in an area near the black hole. The goal of this study was to figure out the structure of the outflow and improve the understanding of the roles black holes play in galaxy formation.

A black hole does not pull gases directly into itself; the gases spin around it reaching higher and higher speeds and temperatures in what is called the accretion disk. These fast-spinning gases create very bright radiation, or light emission, that we see in the form of visible light and x-rays.

When large galaxies are being formed, they need some way to push matter out. "If you don't have something to stop matter flowing into the seeds that make galaxies, they'll get too big," Gerard Kriss, a co-investigator in the study and Hopkins professor, said.

This is accounted for by black holes in the center of active galaxies: While they pull in matter, they also blow some away, pushing nearby gas out and regulating the size of the forming galaxy. The galaxy grows and the accretion, or gravitational attraction, of the black hole becomes stronger, but at the same time the outflow becomes stronger until the galaxy stops growing and the black hole stops blowing out gas. The black hole at the center of our Milky Way galaxy is currently in this sort of inactive state.

Even so, as Kriss explained, there is still information to be obtained from these sorts of black holes, "We could see this gas flowing out . . . but we didn't know exactly how far away from the central black hole it was," he said. "And if you don't know the distance you can't figure out how much total mass is there, or what the energy that's being carried out in the flow is. That's what we were out to measure."

The way they measured this distance was by observing how light coming from the center was absorbed by different ionized elements, which have lost electrons, in gases surrounding the black hole. When the intensity of the black hole changes, the ionizations of the gases change, and they absorb different amounts or different kinds of radiation.

 "Only recently have we had good x-ray telescopes . . . like the XMM-Newton telescope that was the core of these observations, that let us see the ionization state in the various elements that are absorbing the gas and measure their outflow velocities," Kriss explained.

By measuring how quickly the ionizations changed, researchers could infer the density of the gases. With this new detail they could determine the velocity and energy characteristics of the outflowing gases, as well as determine where the outflowing gases originate.

There are different ideas of how these outflows are made around the black hole. Some models say that radiation directly from the accretion disk, which is about the size of a solar system, pushes adjacent gases outwards. If this was the case, some of the gas would be found within the range of a solar system from the black hole. Other models say that the gas comes from a donut-shaped ring of cooler gas that begins a few light-years away from the black hole.

These findings constrain the location of the source of the gas to between 15 and 300 light years from the black hole. This seems to support the theory of a wind of hot gas from outside the accretion disk rather than from the accretion disk itself.

The accretion disk gives off the visible light that we see, but it is not hot enough to create the soft, or low energy, x-ray light that is also emitted from these black holes in active galaxies. These observations found that the amount of visible and x-ray radiation were strongly correlated, suggesting that there is a corona, an area where super-heated energetic gases collect on the accretion disk, through which all of the emitted visible light must pass. Inside the corona, the light picks up some of the energy of the hotter gases, and this boosts the frequency of the radiation up to the soft x-rays that we see emitted from the black hole.

This work gives some precision and support to the current models of the emission from black holes in active galaxies. The full study is published as a series of seven papers in the journal Astronomy and Astrophysics.