due to large intrinsic widths and the relatively low spectral resolving power of R$=$1500. H$\alpha$ and \[NII\]$\lambda$6548,6583 are also blended in most sources. Nonetheless, visual inspection of integrated spectra already shows that the \[NII\]$\lambda$6583/H$\alpha$ ratios are generally below unity, and often below 0.5, unlike in AGN host galaxies at low redshift. We will come back to this point in §\[ssec:diagnostics\] and §\[ssec:metallicity\].

Diagnostic diagrams and gas-phase metallicities {#ssec:diagnostics}
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The bright, rest-frame optical emission lines contain a multitude of constraints on the gas properties, including electron temperatures and densities, ionization parameter, extinction, and gas-phase metal abundance. Taken together, combinations of these lines provide observationally very convenient diagnostics of the gas heating mechanism by young stellar populations or active galactic nuclei [e.g., @veilleux87; @kewley01; @kauffmann03; @kewley06].

Identifying HII regions and AGN narrow-line regions in galaxies by means of their bright optical line ratios is a very successful tool for galaxy evolution studies at low redshift, where the relative uniformness of most HII and AGN narrow-line regions results in fairly narrow sequences in diagnostic diagrams of, e.g., the \[NII\]/H$\alpha$ and \[OIII\]/H$\beta$ line ratios [“BPT-diagrams” @veilleux87; @kewley01; @kewley06].

![ \[fig:metallicity\] BPT diagram of our sources with two of the models of @kewley13a shown 