Double-double radio galaxies: probing duty cycles in AGN and the cocoons of powerful radio sources

Recently, we have identified a small number of extended radio sources that consist of two double-lobed radio sources, which axe well aligned and centered on a common nucleus. We have called these 'Double-double radio galaxies' (DDRGs). The observed structures strongly suggest interrupted central activity as the origin of these sources. We have developed a model based on this assumption with which we are able to explain several observed properties. Since the inner structures of the DDRGs advance inside the cocoon originally formed by the outer lobes, these allow us to probe the cocoon medium. Our results indicate that the cocoon density must be higher than has been assumed in earlier models