In 1978 Bruce Slee was the first to investigate the diffuse radio
sources in galaxy clusters. These are now known as the "relic radio
sources" and this has become a big aspect of the study of radio emission
from clusters of galaxies. An example is the highly cited research is this
paper on "Four Extreme Relic Radio Sources in Clusters of Galaxies"
by Slee, Roy, Murgia, Andernach & Ehle
(2001, AJ 122,
1172). Here Slee et al. describe the results of the highest-resolution
radio observations yet made of four relic radio sources in the Abell clusters
A13, A85, A133, and A4038 using the Very Large Array. The 1.4 GHz images shown
above (with 4" resolution and a noise level of 20 μJy/beam) show a
remarkable variety of fine structure in the form of spectacular arcs, wisps,
plumes, and loops.