2. SYSTEMATICS

2.1. Ingredients

The morphological description of extended extragalactic radio sources conventionally identifies the following components (precise definitions are taken from Bridle & Perley (1984) and Bridle et al. (in preparation)):

Central component This is an unresolved component coinciding to within the observational errors with the nucleus of the associated galaxy or quasar. Most, if not all, central components appear to be the optically-thick bases of jets (see below and Marscher, these proceedings) and the definition is therefore resolution and frequency-dependent.

Jet A jet is:

1. at least 4 times longer than it is wide;
2. separable at high resolution from any other extended structure;
3. aligned with the nucleus of the parent object where it is closest to it. The presumption is that a jet traces the path of a collimated flow or "beam".

Hot-spot Powerful radio sources almost all have bright, compact regions or "hot-spots" close to the outer ends of their structures. The apparent complexity of the following definition results from an attempt to differentiate between bright parts of jets (knots) and features which represent the termination or major disruption of a jet (hot-spots). A hot-spot must:

1. be the brightest feature in the lobe
2. have a surface brightness that is more than four times greater than that of the surrounding emission and
3. have a linear FWHM (after deconvolving the synthesized beam) that is a small fraction (<2%, for example) of the largest diameter of the source.

   If a jet is detected then we add the additional condition:

4. that the hot-spot must be further from the nucleus than the end of the jet, which is defined by:
   1. its disappearance
   2. an abrupt change of direction (i.e. by at least 30° within a knot diameter) or
   3. decollimation by more than a factor of two (as indicated by the widths of any ridge-like or knot-like features along the putative path of any continuing flow).

Lobe The definitions of central component, jet and hot-spot refer to the structure of highest surface-brightness in a radio source. The remaining emission (if any) on a given side of the central component is termed a lobe. Lobes are commonly divided into bridges and tails:

Bridge A bridge is a lobe in which more than half of the emission is between the end of a jet (or hot-spot, if no jet is detected) and the nucleus of the associated galaxy. Synchrotron spectra of bridges show increased curvature towards the nucleus and it is therefore likely that they are formed from electrons left behind at or flowing back from the region in which the jet interacts with the external medium.

Tail (or plume). A tail is a lobe whose emission is mostly further from the nucleus than the end of a jet or the position of a hot-spot. The synchrotron spectra of tails show increasing curvature away from the nucleus, unlike those of bridges, the inference being that they are flowing outwards.