Dying Radio Sources in Clusters
  We present the study of five `dying' nearby radio galaxies belonging to the
WENSS minisurvey and to the B2 bright catalogs: WNB1734+6407, WNB1829+6911,
WNB1851+5707, B2 0120+33, and B2 1610+29. These sources have been selected on
the basis of their extremely steep broad-band radio spectra. The modeling of
the integrated spectra and the deep spectral index images obtained with the VLA
confirmed that in these sources the central engine has ceased to be active for
a significant fraction of their lifetime although their extended lobes have not
yet completely faded away. We found that WNB1851+5707 is in reality composed by
two distinct dying galaxies, which appear blend together as a single source in
the WENSS. In the cases of WNB1829+6911 and B2 0120+33, the fossil radio lobes
are seen in conjunction with a currently active core. A very faint core is
detected also in a MERLIN image of WNB1851+5707a, one of the two dying sources
composing WNB1851+5707. We found that all sources of our sample are located (at
least in projection) at the center of an X-ray emitting cluster. Our results
suggest that the duration of the dying phase for a radio source in cluster can
be significantly higher with respect to that of a radio galaxy in the field.
The simplest interpretation is that the low-frequency emission from the fading
radio lobes last longer if their expansion is somewhat reduced or even stopped.
Another possibility is that the occurrence of dying sources is higher in galaxy
clusters. Radio sources in dense environment, like e.g. the center of cooling
core clusters, may have a peculiar accretion mode which results in a bursting
duty cycle sequence of active and quiescent periods. This result could have
important implications for theories of the life cycles of radio sources and AGN
feedback in clusters of galaxies but awaits confirmation from future
observations of larger samples of objects.
