Space telescopes reveal secrets of turbulent black hole

29 September 2011

Turbulent winds of hot gas swirl around the supermassive black hole. Some of the hot gas is spiraling inward towards the black hole, but some of it is also blown away (Image courtesy NASA/CXC/M.Weiss)

At the centre of many galaxies are supermassive black holes which have the mass of millions of stars. The huge gravitation attraction of these black holes means that they pull in surrounding material. Some of the material disappears into the black hole, but some is ejected in turbulent outbursts. An international team of astronomers, including scientists at UCL’s Mullard Space Science Laboratory (MSSL), has revealed some striking new features of such an outburst around a supermassive black hole in a distant galaxy. They found a very hot corona hovering above the black hole where ultraviolet light is converted into X-rays, and cold gas 'bullets' speeding outwards with velocities up to 700 km/s.

A rotating disc of infalling gas and dust forms around black holes. Unlike popular belief, not all the matter around a black hole is swallowed up though. On the journey inwards the gas and dust emit large amounts of X-ray and ultraviolet radiation. This radiation can be so strong that it diverts a part of the gas inflow into winds flowing outward with velocities up to several hundreds of km/s. An international team of astronomers led by Dr Jelle Kaastra from the SRON Netherlands Institute for Space Research took the opportunity to observe and map the extreme environment around one of the brightest supermassive black holes known to us. This supermassive black hole - in the distant galaxy Markarian 509 - has a mass 300 million times that of the Sun.

Edited Hubble Space Telescope image of Markarian 509. The core of the galaxy hosting the supermassive black hole is much brighter than the rest of the galaxy. Strong winds are emitted from the central region (Image courtesy MAST/SRON).

A disc of gas shining brightly in ultraviolet and X-ray light surrounds the Markarian 509 black hole. This emission varies over time and much more energy is emitted at these wavelengths than in the visible part of the spectrum. 

"The only way to explain this is by having gas hotter than that in the disc, a so-called 'corona', hovering above the disc", Missagh Mehdipour, one of the UCL MSSL astronomers, says. "This corona absorbs and reprocesses the ultraviolet light from the disc, energising it and converting it into X-ray light. It must have a temperature of a few million degrees. Using five space telescopes, which enabled us to observe the area in unprecedented detail, we actually discovered such hot 'corona' of gas hovering above the disc. This discovery allows us to make sense of some of the observations of active galaxies that have been hard to explain so far."

One of the telescopes used in this work is XMM-Newton, parts of which were built by UCL’s MSSL. The X-ray spectrum obtained with XMM-Newton is the best obtained so far of such a system. It reveals unprecedented details of its gaseous environment. For the first time it has been possible to show that the outflow consists of at least five distinct components with temperatures ranging between 20,000 to a million degrees. An ultraviolet spectrum was obtained by the Cosmic Origins Spectrograph of the Hubble Space Telescope. These data reveal that the coolest gas in the line of sight towards Markarian 509 has 14 different velocity components at various locations in the innermost parts of this galaxy. Thus far only seven velocity components were identified.

The combined X-ray and ultraviolet measurements demonstrate that most of the visible outflowing gas is blown off from a dusty gas torus surrounding the central region more than 15 light years away from the black hole. This outflow consists of dense, cold blobs or gas bullets embedded in hotter diffuse gas. "Even at a distance of 15 light years, the energy released near the black hole manages to blow off gas from the dusty torus that surrounds the disc of infalling gas," Kaastra says.

Further outwards, the signatures of the interstellar gas of the host galaxy are seen. That gas is strongly ionised by the central X-ray source: atoms are stripped of some or most of their electrons when illuminated by the powerful flux of X-rays. Even further out, at hundred thousands of light years, the X-ray light shines through gas falling in towards Markarian 509 with speeds of 200 km/s. This gas may point at a collision with a smaller galaxy in the past, that may have triggered the activity of Markarian 509.

Five large space telescopes were involved in this hundred days campaign that took place in late 2009. The heart of the campaign consisted of repeated visible, X-ray and gamma-ray observations with ESA's XMM-Newton and INTEGRAL satellites, which monitored Markarian 509 for six weeks. This was followed by long observations with NASA's Chandra X-ray satellite, using the Low Energy Transmission Grating, and the NASA/ESA Hubble Space Telescope using the new Cosmic Origins Spectrograph. Prior to these observations short snapshots to monitor the behaviour of the source at all wavelengths were taken with the Swift satellite (for which UCL MSSL provided a UV-optical telescope).

The combined efforts of all these instruments and astronomers gave an unprecedented insight into the core of an active galaxy.  Right in the middle of the campaign the source went into outburst. The physical changes due to this outburst could be followed over the electromagnetic spectrum from visible light to X-rays.

The international consortium responsible for this campaign consists of 26

astronomers from 21 institutes on 4 continents. The first results of this campaign are published as a series of 7 papers in Astronomy and Astrophysics (they can be found at the following links:

paper I: http://www.aanda.org/10.1051/0004-6361/201116869

paper II: http://www.aanda.org/10.1051/0004-6361/201116870

paper III: http://www.aanda.org/10.1051/0004-6361/201116899 

paper IV: http://www.aanda.org/10.1051/0004-6361/201116875

paper V: http://www.aanda.org/10.1051/0004-6361/201117067

paper VI: http://www.aanda.org/10.1051/0004-6361/201117123 

paper VII: http://www.aanda.org/10.1051/0004-6361/201117304 

More results are in preparation. 

For more information please contact Graziella Branduardi-Raymont (01483 204133, gbr [a t] mssl.ucl.ac.uk) or Missagh Mehdipour (01483 204284, mmp2 [a t] mssl.ucl.ac.uk). 

You can also take a look at the Dutch PI Website: www.sron.nl

Image captions:- 

Top image: Turbulent winds of hot gas swirl around the supermassive black hole. Some of the hot gas is spiraling inward towards the black hole, but some of it is also blown away (Image courtesy NASA/CXC/M.Weiss).

Second image: Edited Hubble Space Telescope image of Markarian 509. The core of the galaxy hosting the supermassive black hole is much brighter than the rest of the galaxy. Strong winds are emitted from the central region (Image courtesy MAST/SRON).

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