The psi_IPS - LAS relation for extragalactic radio sources

Metre-wavelength IPSobservations give the overall angular sizes psi of scintillating compact structures in radio sources. From 326.5-MHz data for a sample of faint (Ooty) radio sources log psi vs log (largest angular size) is seen, on average, to have slope 0.2, significantly less than 1. A similar trend is seen from 81.5-MHz IPS data for a sample of strong, powerful (3CR) double sources, although the slope is 0.4 and the mean psi about 4 times larger. The difference in slopes is due mainly to the large spread in redshifts of the 3CR sources compared to the expected narrow range for the Ooty sources, while the difference in mean psi values is due to the different methods of determining psi for the two samples, the different frequencies used for the IPS observations and the different mean LAS values. If psi represents hotspot size, the log psi - log LAS correlation has a bearing on the collimation of extended jets in powerful double radio sources. A similar relation is derived from direct observations of hotspots in such sources chosen to minimize projection effects has been found to have slope 0.7. We show that projection effects are partly responsible for the small value 0.4 of the slope of the log psi - log LAS correlation. Blending effects in the IPS observations probably account for the remaining flattening.