Annu. Rev. Astron. Astrophys. 1980. 18: 165-218 Copyright © 1980 by Annual Reviews. All rights reserved

2.3. Overall Linear Sizes of Sources

The linear sizes of extended extragalactic sources span on enormous range, from sources like 3C346 (Pooley & Henbest 1974) which are no larger than their parent galaxies to giant sources like 3C236 with a total size in its largest direction of 4 Mpc (Willis et al. 1974, Strom & Willis 1979). The size distribution and its dependence on source structure and luminosity have been considered by Gavazzi & Perola (1978) using well-defined 3C and B2 samples. To minimize cosmological effects and ensure completeness they restricted their study to (104) elliptical galaxies with z < 0.1. Unfortunately, this inevitably excluded the highest luminosity sources, which tend to be the most distant. The luminosities considered ranged from P₁₇₈ ~ 10^23.4 to ~ 10²⁷ W Hz^-1.

The sizes of edge-brightened doubles are well defined. The linear size function for these sources indicates a slight increase of size with luminosity ( P^1.4), with a median value of ~ 170 kpc at 10²⁵ W Hz^-1. Most have sizes between 150 and 300 kpc with a few ( 5%) that exceed a megaparsec. Only 10% of the two-sided edge-brightened sources are smaller than 130 kpc, suggesting that either the outward motion of source components decelerates rapidly after reaching this size or that the smaller sources are effectively invisible. The one-sided sources (Section 2.1.1) appear to be smaller than the symmetric ones, with typical sizes of a few tens of kpc (Readhead et al. 1978a).

The situation for the more diffuse (Class I) sources is unclear. The median size (~ 150 kpc) of the relaxed doubles and the sizes of their largest members are comparable with those of the edge-brightened doubles. In the data of Gavazzi & Perola there is no significant dependence of the total linear size on luminosity for the diffuse sources, but the uncertainties are large. Because of their ill-defined extremities, estimates of the total sizes of the Class I sources are affected by the sensitivity of measurement. However, sometimes the diffuse sources have regions of enhanced emission within their lobes and the separation of these hot spots provides a less subjective yardstick than the total linear extent of the source. The linear distance of those hot spots from their parent nuclei seems to increase with source luminosity (Birkinshaw et al. 1978). In view of the similar results for sources of diverse morphologies the increase in separation of the hot spots with the total source luminosity is quite well established. Such an effect would be intuitively expected, for example, if the more powerful sources are associated with more powerful nuclear ejecta or beams which can penetrate further into the surrounding medium. It should be stressed that great caution should be exercised in interpreting linear size functions since the results often depend on whether the function is plotted linearly or logarithmically (Ekers & Miley 1977).