Observations by the two of the European Space Agency's space observatories have provided a multi-wavelength view of the mysterious galaxy Centaurus A. The new images, from the Herschel Space Observatory and the XMM-Newton x-ray satellite, are revealing further hints about its cannibalistic past and energetic processes going on in its core.

At a distance of around 12 million light years, Centaurus A is the closest large elliptical galaxy to our own Milky Way. It has been marked as unusual since shortly after its discovery in the 19th century due to a thick lane of dust across its centre - an unusual feature for an elliptical galaxy. But it wasn't until a century later that the galaxy's true nature was revealed.

Emanating from its core are two massive jets of material streaming from a massive black hole in the heart of Centaurus A. When observed by radio telescopes, the jets stretch for up to a million light years, though the Herschel and XMM-Newton results focus on the inner regions.

"Centaurus A is the closest example of a galaxy to us with massive jets from its central black hole," explained Prof Christine Wilson of McMaster University, Canada, who is leading the study of Centaurus A with Herschel. "Observations with Herschel, XMM-Newton and telescopes at many other wavelengths allow us to study their effects on the galaxy and its surroundings."

Strong radio emission is caused by electrons travelling at close to the speed of light through strong magnetic fields, and is so bright that the jets are even visible in the far-infrared images from the Herschel Space Observatory. As well as the jets, the images from this infrared observatory also show a twisted disc of dust near the galaxy's centre.

This odd shape is strong evidence that Centaurus A underwent a cosmic collision with another galaxy in the distant past. The colliding galaxy was ripped apart to form the warped disc, and the formation of young stars heats the dust to cause the infrared glow seen by Herschel.

Such collisions often result in shells and rings of gas and dust, and Centaurus A is no exception. Herschel observations have now confirmed the presence of two clumps of dust that seem to be lined up with the two lobes of the jets.

"The apparent alignment of two clumps with the two jets now seems to be a cosmic coincidence, and it appears that the dust originated from one of the colliding galaxies." explained Dr Robbie Auld, of Cardiff University. "Unlike most dust Herschel sees, which is heated by nearby star formation, the dust in these clumps is being heated by old stars in Centaurus A itself, up to 50,000 light years away."

In x-rays the effect of the two jets of material is clearly visible. Showing the presence of extremely hot gas, the images from the XMM-Newton x-ray satellite clearly show the axis of the one of the jets. While the other jet itself is not seen in by XMM-Newton, the gas it is ploughing into is shocked and heated to very high temperatures, creating a bright x-ray glow.

"XMM-Newton is the observatory most suited to detecting extended weak X-ray emission, often allowing us to see halos around galaxies for the first time," notes Norbert Schartel, XMM-Newton Project Scientist.

In the centre of the galaxy, the massive black hole is also having an effect on its immediate surroundings. The material around it glows brightly in x-rays, but there Herschel has identified an apparent deficit of dust within a few thousand light years of the black hole.

"This could be due to intense x-rays destroying the tiny dust grains, or due to the way the warped ring of dust is affecting star formation" said Prof Wilson. "Either way, Centaurus A is the ideal place to study the extreme processes that occur near super-massive black holes".

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Images

The elliptical galaxy Centaurus A at visible, far-infrared and x-ray wavelengths. In visible light the galaxy appears as a ball of stars, with a thick lane of dust running across it. The far-infrared light shows the glow from jets of material emanating from near the black hole in the galaxy's core. Also visible is a twisted disc of dust, the remnants of a galaxy that was swallowed up in the galaxy's distant past, and two clumps of dust in the top-left and bottom-right corners. In x-rays the jets become visible, as well as the x-ray glow from the super-heated material that they are ploughing in to. Image credits: Far-infrared: ESA/Herschel/PACS/SPIRE/C.D. Wilson, MacMaster University, Canada; X-ray: ESA/XMM-Newton/EPIC; visible: ESO/MPG 2.2-m telescope on La Silla

A far-infrared view of Centaurus A from the Herschel Space Observatory. The twisted disk of material in the centre is clearly visible, the remnants of a galaxy that was swallowed up long ago, and also two clumps of dust in the top left and bottom right corners. The two plumes are the glow from material spewing out from the black hole in the centre of the galaxy. Image credit: Far-infrared: ESA/Herschel/PACS/SPIRE/C.D. Wilson, MacMaster University, Canada; X-ray: ESA/XMM-Newton/EPIC

A composite view of the galaxy Centaurus A in visible light (white), far-infrared (red), and x-rays (blue). In visible light the galaxy appears as a ball of stars, with a thick lane of dust running across it. The far-infrared light shows a glow from jets of material emanating from near the black hole in the galaxy's core. Also visible is a twisted disc of dust, the remnants of a galaxy that was swallowed up in the galaxy's distant past, and also two clumps of dust in the top left and bottom right. In x-rays the jets become clearly visible, along with the x-ray glow from the super-heated material that they are ploughing in to. Image credits: Far-infrared: ESA/Herschel/PACS/SPIRE/C.D. Wilson, MacMaster University, Canada; X-ray: ESA/XMM-Newton/EPIC; visible: ESO/MPG 2.2-m telescope on La Silla

Notes for editors

Herschel

Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA. It was launched in May 2009.

SPIRE

The SPIRE instrument contains an imaging photometer (camera) and an imaging spectrometer. The camera operates in three wavelength bands centred on 250, 350 and 500 μm, and so can make images of the sky simultaneously in three sub-millimetre colours. The SPIRE instrument has been built, assembled and tested in the UK at The Rutherford Appleton Laboratory in Oxfordshire by an international consortium from Europe, US, Canada and China, with strong support from the Science and Technology Facilities Council.

SPIRE has been developed by a consortium of institutes led by Cardiff Univ. (UK) and including: Univ. Lethbridge (Canada); NAOC (China); CEA, LAM (France); IFSI, Univ. Padua (Italy); IAC (Spain); Stockholm Observatory (Sweden); Imperial College London, RAL, UCL-MSSL, UKATC, Univ. Sussex (UK); and Caltech, JPL, NHSC, Univ. Colorado (USA). This development has been supported by national funding agencies: CSA (Canada); NAOC (China); CEA, CNES, CNRS (France); ASI (Italy); MCINN (Spain); SNSB (Sweden); STFC, UKSA (UK); and NASA (USA).

PACS

PACS is also an imaging photometer (camera) and an imaging spectrometer. The camera operates in three bands centred on 70, 100, and 160 μm, respectively. PACS has been developed by a consortium of institutes led by MPE (Germany) and including UVIE (Austria); KUL, CSL, IMEC (Belgium); CEA, OAMP (France); MPIA (Germany); IFSI, OAP/AOT, OAA/CAISMI, LENS, SISSA (Italy); IAC (Spain). This development has been supported by the funding agencies BMVIT (Austria), ESA- PRODEX (Belgium), CEA/CNES (France), DLR (Germany), ASI (Italy), and CICT/MCT (Spain).

UK role in Herschel

The Herschel programme forms a key part of the UK Space Agency's space science programme. A key component of the satellite was led by the UK. The SPIRE (Spectral and Photometric Imaging Receiver) instrument has been developed by an international consortium. It is led by a Principal Investigator from Cardiff University.

The assembly and testing of SPIRE took place at the STFC Rutherford Appleton Laboratory (RAL) in Oxfordshire. The instrument was delivered for installation in the Herschel satellite in April 2007. Other UK institutes involved in SPIRE are Imperial College London, University College London's Mullard Space Science Laboratory and the UK Astronomy Technology Centre, Edinburgh. The UK SPIRE team has lead responsibility for instrument operations.

UK companies involved in the mission include AEA Technology, Analyticon, BOC Edwards, Datasat, MT Satellite Products and System International.

UK Space Agency

The UK Space Agency is at the heart of UK efforts to explore and benefit from space. It is responsible for all strategic decisions on the UK civil space programme and provides a clear, single voice for UK space ambitions.

The UK Space Agency is responsible for ensuring that the UK retains and grows a strategic capability in the space-based systems, technologies, science and applications. It leads the UK's civil space programme in order to win sustainable economic growth, secure new scientific knowledge and provide benefits to all citizens.

The UK Space Agency:

Co-ordinates UK civil space activity

Encourages academic research

Supports the UK space industry

Raises the profile of UK space activities at home and abroad

Increases understanding of space science and its practical benefits

Inspires our next generation of UK scientists and engineers

Licences the launch and operation of UK spacecraft

Promotes co-operation and participation in the European Space programme

Contact details

Dr Robbie Auld

School of Physics and Astronomy

Cardiff University

Email: Robbie.auld [@] astro.cf.ac.uk

Tel: +44 (0)2920 875 106

Prof Christine Wilson

Department of Physics and Astronomy

McMaster University

Email: Wilson [@] physics.mcmaster.ca

Tel: +1 (0)905 525 9140 x27483

Dr Chris North

UK Herschel Outreach Officer

School of Physics and Astronomy

Cardiff University

Email: chris.north [@] astro.cf.ac.uk

Tel: +44 (0)2920 870 537

Madeleine Russell

Press Officer

UK Space Agency

Email: madeleine.russell@ukspaceagency.bis.gsi.gov.uk

Tel: +44 (0)1793 418069