Unbiased studies of diffuse extragalactic radio sources

We present an overview of unbiased studies of diffuse extragalactic radio sources. We use a previously developed filtering technique to remove compact sources from large surveys such as WENSS, WISH, and NVSS and examine the residual diffuse emission. A search of these residuals, unbiased by optical or X‐ray selection, has uncovered a wide variety of diffuse sources, including those associated with groups of galaxies, “blank” fields, and previously unrecognized diffuse radio galaxy emission, as well as halos and relics of rich clusters. A second, targeted survey of the brightest X‐ray clusters results in a number of new sources, and illuminates some important selection effects. When the well‐established relationship between X‐ray and radio halo luminosities is extrapolated to lower levels, the apparent correlation holds quite well for any type of diffuse source, even background noise. The observed correlations must therefore be scrutinized for possible selection effects before physical interpretations are made. We briefly mention a wide‐field mapping project to search for large‐scale structures, and goals for the next generation of studies in magnetic field ‐ angular size parameter space. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)