  We have analyzed the ultraviolet and optical emission line ratios of a large
sample of extragalactic radio sources (QSOs and radio galaxies), with the help
of models combining AGN photoionization and shocks. The results strongly
suggest that the two ionizing mechanisms frequently coexist. The model
sequences obtained by varying the balance between shocks and AGN
photoionization account for most emission line data in the 12 line ratio
diagrams we have considered. In the frequently used diagrams involving
[OIII]5007/Hbeta, [OI]6300/Halpha, [NII]6584/Halpha and [SII]6716,6731, the
effect of varying the shock-photoionization balance mimics a variation of the
ionization parameter (U) in traditional photoionization sequences. In most of
the remaining diagrams, such as [OI]6300/[OIII]5007 vs.[OIII]4363/[OIII]5007,
[OIII]5007/H vs. [OIII]4363/[OIII]5007 and CIII]1909/[CII]2326 vs.
CIV1549/[CII]2326, the data can only be accounted for if both photoionization
and shocks contribute to the line fluxes. The coexistence of shocks and AGN
photoionization also provides an explanation for the most extreme objects in
the NV1240/HeII1640 vs. NV1240/CIV1549 diagram without requiring largely
super-solar metallicities. In addition, we show that there is a relationship
between the [OII]3727/[OIII]5007 ratio and the radio size in radio galaxies.
This strongly supports the hypothesis that the most compact (< 2kpc) and the
largest (> 150 kpc) sources are dominated by photoionization, while
intermediate-sized radio galaxies are dominated by shocks. We briefly discuss
the possible origin of the relation between the shock-ionization balance and
the radio size.
