Starship Asterisk*

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The strange galaxy Centaurus A is pictured in a new image from the European Southern Observatory. With a total exposure time of more than 50 hours this is probably the deepest view of this peculiar and spectacular object ever created. The image was produced by the Wide Field Imager of the MPG/ESO 2.2-metre telescope at ESO’s La Silla Observatory in Chile.

Centaurus A, also known as NGC 5128, is a peculiar massive elliptical galaxy with a supermassive black hole at its heart. It lies about 12 million light-years away in the southern constellation of Centaurus (The Centaur) and has the distinction of being the most prominent radio galaxy in the sky. Astronomers think that the bright nucleus, strong radio emission and jet features of Centaurus A are produced by a central black hole with a mass of about 100 million times that of the Sun. Matter from the dense central parts of the galaxy releases vast amounts of energy as it falls towards the black hole.

This Wide Field Imager (WFI) picture allows us to appreciate the galaxy’s elliptical nature, which shows up as the elongated shape of the fainter outer parts. The glow that fills much of the picture comes from hundreds of billions of cooler and older stars. Unlike most elliptical galaxies, however, Centaurus A’s smooth shape is disturbed by a broad and patchy band of dark material that obscures the galaxy’s centre.

The dark band harbours large amounts of gas, dust and young stars. Bright young star clusters appear at the upper-right and lower-left edges of the band along with the red glow of star-forming clouds of hydrogen, whilst some isolated dust clouds are silhouetted against the stellar background. These features, and the prominent radio emission, are strong evidence that Centaurus A is the result of a merger between two galaxies. The dusty band is probably the mangled remains of a spiral galaxy in the process of being ripped apart by the gravitational pull of the giant elliptical galaxy.

The new set of images from WFI include long exposures through red, green and blue filters as well as filters specially designed to isolate the light from glowing hydrogen and oxygen. The latter help us to spot the known optical jet features around Centaurus A, which were barely visible in a previous image from the Wide Field Imager (eso0315a).

Extending from the galaxy to the upper left corner of the image are two groups of reddish filaments, which are roughly lined up with the huge jets that are prominent in radio images. Both sets of filaments are stellar nurseries, containing hot young stars. Above the left side of the dusty band, we find the inner filaments, lying about 30 000 light-years away of the nucleus. Further out, around 65 000 light-years away from the galaxy’s nucleus and close to the upper left corner of the image, the outer filaments are visible. There is also possibly a very much fainter trace of a counter jet extending to the lower right.

Centaurus A has been extensively studied at wavelengths ranging from radio all the way to gamma-rays. In particular, radio and X-ray observations have been crucial for studying the interaction between the energetic output of the central supermassive black hole and its surroundings, see eso0903. Studies of Centaurus A with ALMA are just beginning.

Many of the observations of Centaurus A used to make this image were taken to see whether it was possible to use ground-based surveys to detect and study variable stars in galaxies like Centaurus A outside the local group. More than 200 new variable stars in Centaurus A were discovered.

Credit: ESO

Zoomable Image

Click to view full size image 1 or image 2

Credit: ALMA (ESO/NAOJ/NRAO); ESO/Y. Beletsky

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A new image of the centre of the distinctive galaxy Centaurus A, made with the Atacama Large Millimeter/submillimeter Array (ALMA), shows how the new observatory allows astronomers to see through the opaque dust lanes that obscure the galaxy’s centre, with unprecedented quality. ALMA is currently in its Early Science phase of observations and is still under construction, but is already the most powerful telescope of its kind. The observatory has just issued the Call for Proposals for its next cycle of observations, in which the growing telescope will have increased capabilities.

Centaurus A is a massive elliptical radio galaxy — a galaxy which emits strong radio waves — and is the most prominent, as well as by far the nearest, radio galaxy in the sky. Centaurus A has therefore been observed with many different telescopes. Its very luminous centre hosts a supermassive black hole with a mass of about 100 million times that of the Sun.

In visible light, a characteristic feature of the galaxy is the dark band that obscures its centre (see for example eso1221). This dust lane harbours large amounts of gas, dust and young stars. These features, together with the strong radio emission, are evidence that Centaurus A is the result of a collision between a giant elliptical galaxy, and a smaller spiral galaxy whose remains form the dusty band.

To see through the obscuring dust in the central band, astronomers need to observe using longer wavelengths of light. This new image of Centaurus A combines observations at wavelengths around one millimetre, made with ALMA, and observations in near-infrared light. It thus provides a clear view through the dust towards the galaxy’s luminous centre.

The new ALMA observations, shown in a range of green, yellow and orange colours, reveal the position and motion of the clouds of gas in the galaxy. They are the sharpest and most sensitive such observations ever made. ALMA was tuned to detect signals with a wavelength around 1.3 millimetres, emitted by molecules of carbon monoxide gas. The motion of the gas in the galaxy causes slight changes to this wavelength, due to the Doppler effect. The motion is shown in this image as changes in colour. Greener features trace gas coming towards us while more orange features depict gas moving away. We can see that the gas to the left of the centre is moving towards us, while the gas to the right of the centre is moving away from us, indicating that the gas is orbiting around the galaxy.

The ALMA observations are overlaid on a near-infrared image of Centaurus A obtained with the SOFI instrument attached to the ESO New Technology Telescope (NTT). The image was processed using an innovative technique that removes the screening effect of the dust (eso0944). We see a clear ring of stars and clusters glowing in a golden colour, the tattered remains of the spiral galaxy being ripped apart by the gravitational pull of the giant elliptical galaxy.

The alignment between the ring of stars seen by the NTT in infrared light and the gas seen by ALMA at millimetre wavelengths highlights different aspects of similar structures in the galaxy. This is an example of how observations with other telescopes can complement these new observations from ALMA.

Credit: ALMA (ESO/NAOJ/NRAO), T.A. Rector (University of Alaska Anchorage).
Visible-light image: ESO.

A NEW IMAGE of the center of the distinctive galaxy Centaurus A, made with the Atacama Large Millimeter/submillimeter Array (ALMA), shows how the new telescope, which is still under construction, allows astronomers to see with unprecedented quality through the opaque dust lanes that obscure the galaxy's center.

Centaurus A is a massive elliptical radio galaxy (a galaxy that emits strong radio waves) and is the most prominent, as well as the nearest, radio galaxy in the sky. Its very luminous center hosts a supermassive black hole with a mass of about 100 million times that of the Sun.

As seen in visible light, a dark dusty band obscures the galaxy's center. This dust lane harbors large amounts of gas, dust and young stars. These features, together with the strong radio emission, indicate that Centaurus A is the result of a collision between a giant elliptical galaxy and a smaller spiral galaxy.

To see through the obscuring dust in the central band, astronomers need to observe using longer wavelengths of light, such as infrared light or radio waves.

The new ALMA observations, shown in a gradation of blue in this image, reveal the position and motion of clouds of gas in the galaxy. They are the sharpest and most sensitive such observations ever made. ALMA was tuned to detect signals with a wavelength around 1.3 millimeters, emitted by molecules of carbon monoxide gas. The motion of the gas in the galaxy causes slight changes to this wavelength, shown in this image as changes in color. Violet and dark-blue features trace gas coming towards us, while light-blue features depict gas moving away. We can see that the gas to the left of the center is moving towards us, while the gas to the right of the center is moving away from us, indicating that the gas is orbiting around the galaxy.

The millimeter wavelength observations were made using super-sensitive radio receivers built by the National Radio Astronomy Observatory. In this image the ALMA observations are overlaid on an optical image of Centaurus A produced by the MPG/ESO 2.2-meter telescope at ESO's La Silla Observatory in Chile.

Click to play embedded YouTube video.

Object: The giant elliptical galaxy Centaurus A
Distance: About 12 million light years

Centaurus A was facing a midlife crisis. The giant elliptical galaxy's brightest stars were old and puffy, and it had nearly run out of gas needed to create new ones. The galaxy was just a featureless blob that had lost its sparkle.

Then a chance encounter allowed boring old Centaurus A to have a fling with a younger, smaller galaxy. The event revived the elder partner, triggering a fresh round of star birth and creating one of its most notable features: a dark dust lane along its middle.

In a surprise twist, new observations show the cosmic hanky-panky also caused Centaurus A to sprout two spiral arms – something no other elliptical galaxy is known to have. The discovery offers new insights into how galaxies form and evolve, and hints at a new way for spiral structure to emerge.

Shocking revelation

The bisecting dust lane led astronomers in the early 19th century to think that Centaurus A might be two separate objects lying side by side. More recent studies have shown that the dust is most likely a disc left behind by a galactic merger.

By blocking visible light, the dust also conceals the intimacies of the galaxy's steamy affair. To gather more clues, Daniel Espada of the National Astronomical Observatory of Japan and colleagues looked at Centaurus A in radio wavelengths.

These longer waves emerge from carbon monoxide gas at the galaxy's centre and can pierce the dusty veil, allowing the team to trace otherwise hidden structures. What they saw was shocking.

"We were quite surprised to find what clearly looked like spiral arms," says team member Alison Peck of the Joint ALMA Observatory in Santiago, Chile. Their images show the tentacles of gas curving around the galaxy's middle, with widths and orientations similar to those of the arms of spiral galaxies like our Milky Way.

What's more, the gas tentacles are "moving in a way that you would expect spiral arms to move", says Peck.

Gassy tendrils

Bruce Elmegreen, a spiral-structure expert at the IBM Research Division in Yorktown Heights, New York, says the arms on Centaurus A are unique because they are made of molecular gas instead of stars, the main components of the Milky Way's arms.

"It's really unusual," says Elmegreen. Usually a disc of gas around an object rotates in such a way that the gas clumps up rather than forming long filaments.

He suspects that Centaurus A is so massive and its stars are so centrally concentrated that the galaxy's rotating core creates a huge shear effect, one that sculpted the gas into spiral structures.

Since galaxies grow via mergers, the new study suggests it is possible that spiral arms exist on other elliptical galaxies. Unfortunately, most are too far away for us to get a detailed peek.

• Astrophysical Journal Letters 756(1) L10 (2012 Sep 01) DOI: 10.1088/2041-8205/756/1/L10
  arXiv.org > astro-ph > arXiv:1208.0404 > 02 Aug 2012

[i]The giant elliptical galaxy M87 shows a split personality because it hides a gaseous spiral at its core. When M87 collided with a spiral galaxy 300 million years ago, it slurped up the spiral's gases, which formed a new spiral inside the larger galaxy.[/i] [url=http://www.eso.org/public/images/eso0005b/][i](Credit: ESO)[/i][/url]

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Most big galaxies fit into one of two camps: pinwheel-shaped spiral galaxies and blobby elliptical galaxies. Spirals like the Milky Way are hip and happening places, with plenty of gas and dust to birth new stars. Ellipticals are like cosmic retirement villages, full of aging residents in the form of red giant stars. Now, astronomers have discovered that one well-known elliptical has a split personality. Centaurus A is hiding a gassy spiral in its center.

"No other elliptical galaxy is known to have spiral arms," said lead author Daniel Espada (National Astronomical Observatory of Japan & Harvard-Smithsonian Center for Astrophysics). "Centaurus A may be an old galaxy, but it's still very young at heart."

Centaurus A isn't your typical elliptical to begin with. Its most striking feature is a dark dust lane across its middle - a sign that it swallowed a spiral galaxy about 300 million years ago.

Centaurus A slurped that galaxy's gases down, forming a disk that we see nearly edge on. From our point of view, any features in that disk have been hidden by the intervening dust.

To tease out the disk's structure, Espada and his colleagues used the sharp vision of the Smithsonian's Submillimeter Array. This radio telescope can see through dust to pick up signals from naturally occurring carbon monoxide gas. By mapping the gas, the team unveiled two distinct spiral arms within the galaxy's core.

These gaseous tendrils have sizes and shapes similar to spiral arms in galaxies like the Milky Way. Also like the Milky Way's spiral arms, they are forming new generations of stars.

"Centaurus A has been given a new lease on life by that past merger," said Espada.

Computer simulations suggest that the spiral features might endure for hundreds of millions of years to come.

Although Centaurus A is the first elliptical galaxy found to have spiral arms, it may not be the last. Since it's only 12 million light-years away, it's relatively nearby and easy to study. The Atacama Large Millimeter/submillimeter Array (ALMA) potentially can find more split-personality galaxies with its improved radio "vision."

"We definitely will use ALMA to search for other objects that are similar to Centaurus A," added Espada.

• Astrophysical Journal Letters 756(1) L10 (2012 Sep 01) DOI: 10.1088/2041-8205/756/1/L10
  arXiv.org > astro-ph > arXiv:1208.0404 > 02 Aug 2012