Daily Image

09-10-2007
PreviousNext
Click here or on the picture for a full size image.

The many lives of radio galaxies

Submitter: Arno Schoenmakers
Description: Many galaxies in the Universe host black holes in their center whose masses exceed 1 - 100 million solar masses. Some of these super-massive black holes are in a phase of high activity, which can lead to the ejection of two beams (''jets'') of relativistic particles, so that a large radio-emitting structure can be formed. These objects are called radio galaxies. The size of radio galaxies can well exceed 1 million parsec (roughly 3 million light-years!) and these are called Giant radio galaxies.
Some giant radio galaxies appear to decrease their jet-forming activity strongly for a while (estimated at a few million years), and then become highly active again. This may lead to a second phase of radio structure formation. We have called such sources double-double radio galaxies (PhD. thesis A. Schoenmakers, 1999). The 'old' radio galaxy will fade away (in roughly 100 million years) as its energy source has been cut off and it will become a so-called 'relic' structure, only detectable at the lowest radio frequencies (which is perfect for LOFAR detection).
Recently we have discovered a radio galaxy which seems to have undergone also a third period of intermittency. This is extremely rare, especially so as the radio emitting structures from the former two periods of activity are still well detectable at GHz-frequencies. The picture shows recent VLA images of these three phases of activity.
What is striking in this picture is the almost perfect alignment of the three structures, despite their size difference of almost a factor of 100. Also, the bright endpoints of the middle set of lobes (so-called ''hotspots'') indicate that the now terminated jet feeding these structures has not yet disappeared completely; compare this to an intermittent fountain where an earlier ejected beam of water has not yet splashed to the ground when the new beam is ejected. This helps us in constarining the age of the smallest inner structure to a few million years, only.
These pictures, and a discussion on the implications for the properties of this remarkable radio source, are presented in a paper now available on astro-ph: http://arxiv.org/abs/0709.4548 (Brocksopp, Kaiser, Schoenmakers, de Bruyn).
Copyright: ASTRON 2007
 
  Follow us on Twitter
Please feel free to submit an image using the Submit page.