Models developed to describe FR IIb and FR IIa radio sources

Several models developed to describe the properties of FR IIa and FR IIb radio sources are compared and contrasted here. The fundamental picture of a classical double (edge-brightened, FR II) radio source powered by highly collimated outflows from a central compact object is common to most models. However, models do differ substantially, and some of the key differences are discussed here. Models must be tested and constrained by comparisons with radio data. The extensive, detailed, radio data that can be used to constrain the physical properties of FR II sources will be described. The radio bridge properties of FR II sources indicates they should be divided into two categories: FR IIa and FR IIb sources. Low-power FR IIa sources are typically found at low-redshift and have distorted radio bridge structure. High-power FR IIb sources are typically found at high-redshift and have fairly regular, 'cigar-like' bridge shapes. It is anticipated that two model formalisms may be needed to describe FR IIb and FR IIa sources. Unlike FR IIa sources, FR IIb sources are found to inhabit gaseous environments similar to those found in low-redshift clusters of galaxies, though the sources have redshifts between zero and two. Thus, FR IIb sources most likely lie in clusters of galaxies. The importance of X-ray measurements to determine the density and temperature of the gas in the vicinity of FR IIb sources is stressed.