Cannibal galaxy watched while "digesting"

Deep insights into the interior of the giant galaxy Centaurus succeeded

The "meal" of Centaurus A © ESO / Y. Beletsky
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Researchers have used a new astronomical image evaluation method on images taken by ESO's 3.58-meter New Technology Telescope (NTT). Result: unparalleled deep insights into the interior of the giant galaxy Centaurus A. In particular, the remains of the "last meal" of this cannibal galaxy were revealed. It is the remnant of a smaller spiral galaxy that Centaurus A has incorporated.

The impressive image also shows thousands of star clusters that populate the central region of Centaurus A, astronomers report in the current issue of the journal "Astronomy and Astrophysics".

Centaurus A (NGC 5128) is the next elliptical giant galaxy with a distance of eleven million light years, and one of the most studied objects in the southern sky. Its unusual appearance had already attracted the attention of the famous British astronomer John Herschel in 1847, who had made a survey of the southern sky and made a comprehensive list of mists.

Opaque dust band

What Herschel could not yet know is that the spectacular look of the galaxy goes back to an opaque band of dust that covers the central area of ​​the galaxy. From today's perspective, this dust is a silent witness to a huge cosmic event: 200 to 700 million years ago, according to today's explanation, this galaxy has incorporated a smaller, gas and dusty spiral galaxy.

Its remnants are still detectable in the central region of Centaurus A, most likely leading to the formation of new generations of stars. Today's models of galaxy formation suggest that large elliptical galaxies are generally formed by this type of fusion process. display

The Infrared Space Observatory (ISO) of the European Space Agency ESA has already provided a first look at these "leftovers". The ISO images showed a structure about 16, 500 light-years wide, reminiscent of a small barred spiral galaxy. More recently, observations with NASA's Spitzer Space Telescope have shown that the structure is more in the shape of a parallelogram-shapes of this sort are expected when a spiral galaxy merges with an elliptical galaxy and becomes distorted.

The "meal" of Centaurus A: images in the visible (left) and in the near infrared light ESO / Y. Beletsky

Researchers look through dust

The now published illustration uses a new method that astronomers can see through the dust. SofI, a combination of an infrared camera and a spectrograph on the 3.58-meter New Technology Telescope at the ESO La Silla Observatory, captured images in three different wavelength ranges of the near infrared - according to the astronomical filters J, H and K. The dust, which obscures the view of the central regions of the galaxy, affects the light in these three wavelength ranges in different ways.

The new method combines the information of the three partial images into a combination image in which the influence of the dust is largely eliminated. The result is an unprecedentedly clear view of the center of Centaurus A.

Starring behind dustbands

The deep insights into the center of the galaxy had surprises in store: "Behind the dustbeams lies a clearly recognizable ring of stars and star clusters, which our pictures now visualize in detail for the first time, " says Jouni Kainulainen of Max Planck Institute for Astronomy and the University of Helsinki, the main author of the journal. A deeper investigation of these structures will now provide information on the course of the merger process, and show what role star formation played in this process.

The research group sees promising application possibilities for the evaluation technique: These are the first steps towards a technique that enables large gas clouds in other galaxies to be detailed and without much technical effort picture, explains co-author Jo o Alves. "Once we know how these clouds form and evolve, we also understand how galaxy star formation is happening."

Co-author Yuri Beletsky looks to the future: This technique is a good supplement to the radio data that [the submillimeter telescope field being built] will deliver ALMA to nearby galaxies. At the same time, it opens interesting possibilities for observing stars in other galaxies with the planned European Extremely Large Telescope (E-ELT) and the James Webb Space Telescope - dust is found in virtually all galaxies.

Black hole lurks in the center of Centaurus A.

Previous observations with the ISAAC instrument on the VLT, ESO's Very Large Telescope at the Paranal Observatory, had confirmed that a supermassive black hole is lurking in the center of Centaurus A. According to the researchers, the mass of this black hole corresponds to around 200 million solar masses, which is 50 times the mass of the black hole in the center of our home galaxy, the Milky Way.

In contrast to the conditions in the center of the Milky Way, the central black hole of Centaurus A is continuously fed with new matter. This makes Centaurus A an active galaxy: it is one of the brightest radio sources in the sky - this is already indicated by the A in the galaxy name - and radio and X-ray observations show that the galactic nucleus contains high-energy particle currents, so-called Jets to be thrown out.

(idw - Max Planck Institute for Astronomy, 23.11.2009 - DLO)