GPS/CSS radio sources and their relation to other AGN

We are entering a new era of sensitive, large-area and multi-frequency radio surveys that will allow us to identify Gigahertz-Peaked Spectrum (GPS) and Compact Steep Spectrum (CSS) radio sources over a wide range in radio luminosity and study them within the context of the overall radio-source populations to which they belong. "Classical" GPS/CSS objects are extremely luminous radio sources with a compact double morphology, commonly thought to represent the earliest stages in the life cycle of powerful radio galaxies (e.g. O'Dea 1998). It is now becoming easier to identify GPS/CSS candidates with much lower radio luminosity -particularly in the nearby Universe. These less powerful objects, with typical 1.4 GHz radio luminosities of 10(23) to 10(25) W Hz(-1), include peaked-spectrum radio sources with a core-jet morphology on parsec scales as well as high-frequency GPS-like peaked components embedded within lower-frequency extended emission. In the latter case, the presence of a young GPS component may not be evident from low-frequency data alone. Many radio galaxies in the local Universe have a compact (FR-0) morphology, and appear to lack extended radio emission on kiloparsec scales. The relationship of these FR-0 objects to the classical GPS/CSS radio sources remains unclear - some of them may represent short-lived episodes of AGN activity that will not lead to an extended FR-1 or FR-2 radio galaxy. Future wide-band radio surveys will shed more light on this - such surveys should ideally be coordinated to cover the full frequency range from 100 MHz to 100 GHz in order to sample all stages of GPS/CSS evolution in an unbiased way. (C) 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim