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A cannibalistic galaxy

05 April 2012

Canabalistic galaxy webElliptical galaxy Centaurus A at visible, far-infrared and x-ray wavelengths. ESA/Herschel/PACS/ SPIRE/C.D. Wilson, MacMaster University.

Cardiff astronomers are amongst a team using the Herschel Space Observatory to view a galaxy that harbours a dark secret. The new images of "Centaurus A" have revealed 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. In the 1840s Sir John Herschel observed a dark streak running across the galaxy, now known to be a thick dust lane.

This is an odd feature for an elliptical galaxy, and marked Centaurus A as unusual, but it wasn't until a century later that its true nature was revealed.

Emanating from its core are two massive jets streaming from a massive black hole in the heart of Centaurus A, which weighs in at the equivalent of 10 million Suns. When observed by radio telescopes, the jets stretch for up to a million light years, though the Herschel results focus on the inner regions.

The Herschel images are combined with results detected by ESA’s XMM-Newton x-ray satellite.

"Centaurus A is the closest example of a galaxy to us with massive jets from its central black hole," explained Professor Steve Eales, School of Physics and Astronomy.

"Observations with Herschel, XMM-Newton and telescopes at many other wavelengths allow us to study their effects on the galaxy and its surroundings," he added.

Strong radio emission is caused by electrons travelling at close to the speed of light through magnetic fields, and is so bright that the jets are even visible in the sub-millimetre images from the Herschel Space Observatory. Named after John Herschel's father, Sir William Herschel, the Herschel Space Observatory shows a twisted disc of dust in its 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 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, School of Physics and Astronomy, who has led one of the studies.

"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," he adds.

In x-rays the effect of the two black hole 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.

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 Herschel has shown that there is an apparent deficit of dust within a few thousand light years of the centre.

"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 Professor Eales.

"Either way, Centaurus A is the ideal place to study the extreme processes that occur near super-massive black holes," he added.

Related links

School of Physics and Astronomy

European Space Agency (ESA)

Herschel Space Observatory