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Zooming in on an intergalactic collision


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Point a camera at a particular patch of sky for more than 50 hours and what do you get? This image of Centaurus A, a galaxy 12 million light years away:

New image of Centaurus A. See bottom of post for link to bigger version. Credit: ESO

Well, for “camera” read (after taking a deep breath) “Wide Field Imager of the MPG/ESO 2.2-metre telescope at the European Southern Observatory’s La Silla Observatory in Chile”. But don’t let that put you off. You can also make out the galaxy with a pair of binoculars – in the night sky you’ll find it in the southern constellation Centaurus.

If you fancy a closer look, and a bit more history, keep on scrolling.

Zooming in on the strange galaxy Centaurus A from ESO Observatory on Vimeo.

Centaurus A, also known as NGC 5128, is a massive peculiar galaxy hiding a supermassive black hole at its heart. (Peculiar is an actual classification, not just an adjective.) A couple of things make it different from a run-of-the-mill galaxy. One is the dark band across its centre. That dark bands is made up of a lot of gas, dust and young stars. Bands like this are typically found in spiral galaxies. This one, however, is warped, making it look patchy and suggesting something funny has gone on in the galaxy’s past.

But that’s not all. If you take a look at the mass of bright stars in the picture, they look a lot like an elliptical galaxy. So which is it?

The answer could well be both. Astronomers believe that Centaurus A is the remnant of a collision on an intergalactic scale. In 2004, the Spitzer telescope revealed that it Centaurus A started off as an elliptical galaxy. This elliptical galaxy happened across a spiral galaxy around 200 million years ago. The result is what you can see above. Once it began devouring the spiral galaxy, Centaurus A twisted the spiral into a parellelogram shape – this was the clue, found by Spitzer, that led astronomers to the their conclusions about the galaxy’s gruesome history.

Centaurus A is one of the most prominent radio galaxies in the sky, and harbours a black hole with a mass around 100 million times that of the Sun. The galactic feeding frenzy it undertook is thought to provide the fuel for the strong radio activity that surrounds that black hole. View Centaurus A through a radio telescope and it looks a little different to the image above: you can see two jets shooting out from its black hole. Those jets are made of high energy streams of matter in which the electrons have been stripped away from their atoms – a plasma, in other words.

If you look closely at the image above, you can see reddish filaments that roughly line up with the black hole’s jets. Those filaments are stellar nurseries full of young stars. You can see them a bit better in the video below.

Panning over a deep view at the strange galaxy Centaurus A from ESO Observatory on Vimeo.

For the biggest version of the image available, head over to ESO where you’ll find a zoomable one which is pretty spectacular.

Kelly Oakes About the Author: Kelly Oakes has a master's in science communication and a physics degree, both from Imperial College London. Now she spends her days writing about science. Follow on Twitter @kahoakes.

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





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  1. 1. Ralf123 5:08 pm 05/19/2012

    The videos don’t play. Why don’t you post images?

    Link to this
  2. 2. Kelly Oakes in reply to Kelly Oakes 6:06 pm 05/19/2012

    Hi Ralf123,
    They work for me, perhaps there’s a problem at your end? In any case, if you want non-moving pictures, you can click the link at the bottom of the post.

    Link to this
  3. 3. callejero 1:01 pm 05/20/2012

    I’m seeing quite a number of objects “strung” together. Does anyone know how common this is and what causes it?

    Link to this
  4. 4. jtdwyer 7:08 am 05/21/2012

    callejero – I think those are the relativistic jets ejecting elementary particles at nearly the speed of light along the (magnetic) poles of the supermassive black hole forming the Active Galactic Nucleus. They’re mentioned in the article – please also see http://en.wikipedia.org/wiki/Relativistic_jets

    Link to this

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