The balance between shocks and AGN photoionization in radio sources and its relation to the radio size

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]λ5007/Hβ, [OI]λ6300/Hα, [NII]λ6584/Hα and [SII]6716, 6731 (Veilleux & Osterbrock 1987), 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]λ6300/[OIII]λ5007 vs. [OIII]λ4363/[OIII]λ5007, [OIII]λ5007/Hβ vs. [OIII]λ4363/[OIII]λ5007 and CIII] λ1909/[CII]λ2326 vs. CIVλ1549/[CII]λ2326, 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 NVλ1240/HeIIλ1640 vs. NVλ1240/CIVλ1549 diagram without requiring largely super-solar metallicities. In addition, we show that there is a relationship between the [OII]λ3727/[OIII]λ5007 ratio (i.e., the ionization level of the gas) 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.