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How `UFOs' Curb Black Hole Growth

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Artist's impression of a galaxy that is releasing material via two particle jets (red/orange) as well as via ultra-fast outflows (grey/blue), both powered by a black hole. Credit: {link url="http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=50098"}ESA/AOES Medialab{/link}

Something unusual has been spotted lurking around several galaxies' central black holes. Astronomers think it may be limiting the growth of the black holes – and stars elsewhere in the galaxies, too.

Astronomers studying nearby galaxies have found a new type of outflow called an ultra-fast outflow, or UFO. An international team of astronomers led by Francesco Tombesi from the University of Maryland and NASA's Goddard Space Flight Centre in Maryland, US, carried out the work. They published their findings in Monthly Notices of the Royal Astronomical Society yesterday.

Astronomers have known for years that the more massive the black hole lying at the centre of a galaxy, the more stars they could expect to find in that galaxy's 'bulge' – a large, roughly spherical region packed with stars.

But they thought it was a little odd that the two were linked. A black hole – though often portrayed as an all-consuming cosmic vacuum cleaner – only really affects the space close to it and on the whole has little impact on the galaxy in which it resides.

Black holes are surrounded by a million degree X-ray emitting disk of gas. This gas is sucked into the black hole and powers it. Some of the disk material, however, escapes and is redirected outwards as a jet of energetic particles.

These jets do not explain the link between black holes and bulges, though. So astronomers went looking for a new kind of black hole outflow that was somewhere between these particle jets, which travel at half the speed of light, and known outflows that travel much slower.

Now, Francesco Tombesi and his colleagues have found that new type of outflow. It is called "ultra-fast" because it is faster than already-known outflows, but slower than jets. UFOs could explain the link between black hole and bulge size. They contain more mass and have a wider opening angle than jets do, so should interact more with the stuff between stars in the galaxy.

Tombesi’s team surveyed 42 nearby galaxies that were known to host black holes using ESA's XMM-Newton X-ray Observatory.

In 40% of galaxies they looked at, Tombesi and his team saw that there must be a cloud of material in between Earth and the black holes. These clouds were changing the properties of the X-rays coming from the black holes’ disks. The team worked out that the clouds of material must be between 0.001 and 0.1 light years away from the black hole. In astronomical terms, that is extremely close.

Tombesi's team has previously published two papers showing that the clouds they saw were a new kind of outflow. The paper published yesterday helps to hone in on the properties of UFOs.

Because of their proximity to the black hole, UFOs get exposed to a vast amount of radiation. This means the particles they are made of have their electrons stripped from their atoms in a process known as ionisation, leaving the UFOs consisting of highly ionised plasma.

UFOs take material from around the black hole, around one solar mass per year, and eject it at speeds of up to 100,000 kilometres per second. That mass could have otherwise fallen into the black hole, or been used in star formation in the galaxy's bulge. By removing this matter from the centre of the galaxy the UFOs stunt both the growth of the black hole and the growth of stars in the bulge.

Tombesi and his team have one more paper is in the works. It will compare UFOs with a slower outflow known as a warm absorber that they believe has a different origin. After that, they will have to wait until the launch of the Astro-H X-ray telescope, scheduled for 2014, to get a better picture of how the UFOs affect their black hole's galaxy.

Reference

Tombesi, F., Cappi, M., Reeves, J., & Braito, V. (2012). Evidence for ultrafast outflows in radio-quiet AGNs - III. Location and energetics Monthly Notices of the Royal Astronomical Society: Letters DOI: 10.1111/j.1745-3933.2012.01221.x

The views expressed are those of the author and are not necessarily those of Scientific American.

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