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Astron. Astrophys. 323, 21-30 (1997)

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7. Low redshift radio galaxies

In the above discussion we used the UV emission lines as a diagnostic to distinguish between shock and AGN photoionization in the EELR of powerful high redshift radio galaxies.

At [FORMULA] redshifts, most of the information we have comes from the optical emission lines. The most recent shock models developed by Dopita and Sutherland (1995, 1996) produce, in general, optical line ratios which overlap with the -1.5 PL predictions. Therefore, the optical emission lines do not provide a clear-cut way to distinguish between the mechanisms.

However, the UV line ratios offer a powerful diagnostic for low redshift radio galaxies with low U values (log [FORMULA] -2.0). For high U values, the distinction is difficult, due to the overlap of the shock and -1.5 power law models (see above), and the diagnostics become inefficient at the high ionization end of the diagram. A comparison of Fig.  1 and 3 reveals that the -1.5 power law with log [FORMULA] -2.0 lie far from the shock models, due to the much fainter CIV emission. While these AGN photoionization models produce [FORMULA] 0.5 and [FORMULA] 0.2, the values for the shock models are [FORMULA] 1.0 and [FORMULA] 0.6 and respectively.

As mentioned before, observations of low redshift radio galaxies with strong jet/cloud interactions show that the emitting gas has a low ionization level (Clark & Tadhunter 1996). The spectrum is dominated by the lines emitted by the "cool" high density (i.e. low ionization parameter) gas behind the shock. Therefore, they are suitable for the diagnostics. A good example is the radio galaxy PKS2250-41. Clark et. al (1996) have presented clear evidence of the presence of a jet/cloud interaction in the EELR of this object both from imaging and optical spectroscopy. The optical line ratios indicate that log [FORMULA]  -3. Thus the measured UV line ratios have the potential to determine which mechanism dominates the emission line processes in the EELR of this object. The possibility of a -1.0 PL ionizing continuum is rejected because it does not explain the optical line ratios: shock and/or -1.5 PL (or hot black body) are the main possibilities.

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© European Southern Observatory (ESO) 1997

Online publication: June 5, 1998