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Extension of article Centaurus A

Observations

Radio waves

Overview over the radio structure of Centaurus A. The range of the structures observable with radio waves is impressive: The whole radio emitting region extends about 1.8 million light years (about 8° degrees in the sky). Through observations with the VLBI technique structures of the jet and the core smaller than a light year could be resolved (corresponding to a resolution of 0.68 x 0.41 milli-arcseconds.)

This view of the jets of Centaurus A was created through observations in radio waves with a wavelength of 20 cm with the VLA. The position of the radio jet and the knots within the jets matches very good with the structures seen in the x-ray jet. This region of the jet is named „Inner Lobe“.

The most of the radio emission from Centaurus A comes from two regions. The outer radio source is symmetric and is rotated about 40 degrees against the inner region.

The outer radio source has a diameter of about 1.8 million light years, making it one of the largest objects of this type in the sky (for comparison: This distance is farther than the separation of the Andromeda Galaxy and our Galaxy). The apparent diameter is about 8 degrees or about sixteen Moon breadths. Although Centaurus A has unusual large radio lobes for a radio galaxy its radio emissions are quite weak - about thousand times stronger than the radio emissions of a spiral galaxy, but in fact only one-thousandth the strenght of the optical emissions of its stars. By its accidental proximity, Centaurus A uniquely allows detailed studies aimed at determining the nature of the galaxy and in particular the origin of the giant radio source it is hosting. Other radio galaxies are much farther away.

The distribution of radio emission across the northern and southern outer lobes is very asymmetric. These radio clouds consists of hot low-density gas, which was ejected from the nucleus. The radio emissions itself is created through fast moving electrons, which are spiraling in the magnetic field of the radio lobes and then emitting synchroton radiation in form of radio waves.

The brightest part of the giant northern lobe is called the northern middle lobe; no such feature is detected in the giant southern lobe. In the frequency of 5 GHz this region contributes about 45% to the total radio emissions. The middle lobe is also associated with soft X-ray emissions. The inner region extends symmetrically in two arms of 16000 length light years from the nucleus. About 30 % of the radio emissions in the 5 GHz frequency is contributed by the inner radio lobes. The radio emissinos of the northern lobe region is about 40 % higher then from the southern lobe. The direction of polarisation of the inner lobes differs from that of the middle and other lobes. Outside of the sharply defined norther plumes in a distance of 6.3 kpc from the nucleous the polarisation changes by 90°.

With the help of the VLBI technique the low resolution of radio telescopes can be increased dramatically. It is done by connecting several telescopes around the world into a virtually one. The structure of the jets in Centaurus A could be observed in detail with the VLBA radio interferometer from 1992 to 2000. Through this observation there were two components discovered (named C1 and C2) which are moving with 12 % the speed of light. Also there was a component observed (C3) which is close to the core and is stationary. Observations at the frequency of 22.2 GHz are showing, that the jet is linear and well collimated on scales as small as 0.02 parsec. The region of initial collimation of the Centaurus A jet likely occurs on angular scales of 100 micro arcseconds. Future space VLBI missions at high frequency may be required to resolve this region.