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

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4. The UV line ratios: the diagnostic diagrams

We have studied three diagnostic diagrams which involve the CIV [FORMULA] 1950, CIII] [FORMULA] 1909 and HeII [FORMULA] 1640 lines which usually dominate (together with Ly [FORMULA]) the UV spectrum of HZRG. We compare the position in the diagrams of the data with the prediction of pure AGN photoionization and shock models. Shock model calculations for the Ly [FORMULA] line do not exist in the literature and we have not included this line in our diagrams (see VMBF96 for a detailed study of the CIV [FORMULA] 1950/Ly [FORMULA] vs. CIV [FORMULA] 1950/CIII] [FORMULA] 1909 diagnostic diagram for the same sample of objects studied here and AGN photoionization models).

The data are represented in the diagrams with hollow circles (van Ojik's data) and solid triangles (other objects). The different models are distinguished by different line types in the diagrams (see figure captions).

Comparing data and model predictions we want to answer the questions: is it possible to disentangle the main mechanism responsible for the ionization of the gas in HZRG using the UV line ratios? Which models agree better with the data: shocks or AGN photoionization?

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© European Southern Observatory (ESO) 1997

Online publication: June 5, 1998