Angular Size-Flux Density Relation for Extragalactic Radio Sources

A comparison has been made of the maximum angular extents of 163 weak extragalactic radio sources observed by the method of lunar occultation at Ootacamund with those for 199 strong sources from the 3CR complete sample. The angular sizes of the 62 sources with flux densities $$S_{408} \geqslant 16 \cdot 5 \,\text{JY} \,(1 \,\text{JY} = 10^{-26} \text{W}\,\text{m}^{-2} \,\text{Hz}^{-1})$$ in the All-sky catalogue of Robertson were also considered. Over a flux density range of about 0.3–1OO Jy at 408 MHz, it is found that the median value of angular size Om increases with flux density S and is related approximately as $$\theta_\text{m} \propto S^{1/2}.$$ A similar relation is found if the identified QSOs are excluded from the source samples. These results indicate that statistically the weaker sources are located farther away. Since both θ and S are dependent on world models, their relationship is of value for cosmological investigations.