The Compact Steep‐Spectrum and Gigahertz Peaked‐Spectrum Radio Sources

I review the radio to X‐ray properties of gigahertz peaked‐spectrum (GPS) and compact steep‐spectrum (CSS) sources, the current hypotheses for their origin, and their use to constrain the evolution of powerful radio galaxies. The GPS and CSS sources are compact, powerful radio sources with well‐defined peaks in their radio spectra (near 1 GHz in the GPS and near 100 MHz in the CSS). The GPS sources are entirely contained within the extent of the narrow‐line region (≲1 kpc), while the CSS sources are contained entirely within the host galaxy (≲15 kpc). The peaks in the spectra are probably due to synchrotron self‐absorption, though free‐free absorption through an inhomogeneous screen may also play a role. The turnover frequency varies with linear size l as νm∝l-0.65, suggesting a simple physical relationship between these parameters. The radio morphologies are strikingly like those of the large‐scale classical doubles, though some sources can have very distorted morphologies suggestive of interactions. Radio polarization tends to be low, and in some cases the Faraday rotation measures can be extremely large. The IR properties are consistent with stellar populations and active galactic nucleus (AGN) bolometric luminosity similar to that of the 3CR classical doubles. The optical host galaxy properties (absolute magnitude, Hubble diagram, evidence for interaction) are consistent with those of the 3CR classical doubles. CSS sources at all redshifts exhibit high surface brightness optical light (most likely emission‐line gas) that is aligned with the radio axis. The optical emission‐line properties suggest (1) interaction of the radio source with the emission‐line gas and (2) the presence of dust toward the emission‐line regions. X‐ray observations of high‐redshift GPS quasars and a couple of GPS galaxies suggest the presence of significant columns of gas toward the nuclei. Searches for cold gas in the host galaxies have revealed large amounts of molecular gas and smaller amounts of atomic gas in several sources, though probably not enough to confine the radio sources. The main competing models for the GPS and CSS sources are that (1) they are frustrated by interaction with dense gas in their environments and (2) they are young and evolving radio sources that will become large‐scale sources. Combining the bright GPS and CSS samples with the 3CR results in a sample spanning a range in source size of 105 that can be used to study source evolution. The number density versus linear size relation is consistent with a picture in which the sources expand with constant velocity and the radio power drops with linear size l according to P∝l-0.5. This strong evolution suggests that at least some of the GPS and CSS sources evolve to become lower luminosity FR 1 radio sources. The GPS and CSS sources are important probes of their host galaxies and will provide critical clues to the origin and evolution of powerful radio sources.