Centaurus A – In to the Heart of Darkness

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The giant black scar of dust crossing the centre of Centaurus A, also known at NGC5128 in visible light all but vanishes in this far infrared image taken by the Herschel Space Observatory, to reveal the heart of this galactic collision, home to intense star forming regions and evidence for one of the Universe’s most destructive forces, a black hole.

Centaurus A is a lenticular (intermediate between spiral and elliptical) galaxy, which exhibits a complex variety of dust, gas and stellar regions. The Herschel image reveals thermal emissions emanating from the dusty central band, which was first seen visibly from the Paramatta observatory in Australia in 1826 by James Dunlop, and then famously by Sir John Herschel in 1847 during his work surveying the southern hemisphere skies for the seminal “Outlines of Astronomy”.

Ignored largely for the next century, due in part to a lack of a large observatory in the Southern hemisphere, it was 1954 when astronomers at Mt Palomar first proposed the theory that its unusual appearance was the result of a collision between two galaxies. The unusually large amounts of dust for a galaxy of this type are indicative of a merger between a large elliptical galaxy with a late type spiral galaxy, rich in both dust and gas

As one of the nearest strong radio galaxies to our own, at a distance of around 12 million light years, it is visible in even modest amateur optical equipment, with the dark band of dust clearly bisecting the main elliptical glow.

The Herschel image of Centaurus A combined from data from both the Photoconductor Array Camera and Spectrometer (PACS) and Spectral and Photometric Imaging Receiver (SPIRE) instruments reveals spectacular detail. Image data from PACS at 100 microns glowing almost golden yellow in the core of the galaxy, where intense star formation is taking place. This parallelogram shaped region of dust can be best described using galaxy formation models where a flat spiral galaxy collides with an elliptical galaxy, and becomes warped in the process.

SPIRE data provide stunning views of the jets emanating from the central region at 250, 350 and 500 microns. Both of the jets, the upper left of which in the image measures approximately 15,000 light years, are clearly seen in the Herschel image. Thought to be powered by a supermassive black hole, these jets vanish in visible wavelengths and are most likely mainly due to synchrotron emission from relativistic electrons spiraling around magnetic field lines in the jets.

This far infrared image, taken as part of the Herschel VNGS (Very Nearby Galaxies Survey) shows the galaxy with unprecedented spatial resolution at these wavelengths, and is key to understanding the physical processes and properties of gas and dust in galaxies.

Credit: ESA/Herschel/PACS/SPIRE/C.D Wilson, MacMaster University, Hamilton, Ontario, Canada

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