Observational Details: Radio

In this chapter we discuss the observations of radio jets. After some introductory remarks, mainly describing the classic Fanaroff–Riley separation of double radio sources, we separate the topic into small-scale (parsec) and large-scale (kiloparsec) jets. These two types of objects have been studied with different telescopes and the two subfields have developed largely independently of each other. Very Long Baseline Interferometry (VLBI) with angular resolution of 1 milliarcsecond (mas) and better is used at parsec (pc) scales, and the Very Large Array (VLA) and similar instruments are used for larger scales. VLBI is sensitive only to high surface-brightness features (≳ 10^5 K) and so typically studies only the core region of a source; whereas the VLA can image the faint outer plumes of a radio galaxy but its core region is subsumed into an unresolved bright spot. This instrumental distinction, however is not definitive, as some observations at certain frequencies cover the intermediate range, e.g., MERLIN and its connections to the European VLBI Network cover a wide range of scales. See Figure 1.1 for a montage of VLBI and VLA images that span the pc–kpc range.