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A trend in photoionization models of the Narrow Line Regions (NLR) of Seyfert 2’s is to consider a wide range of cloud densities (at each NLR radius) that extends from 100 up to 1e8 cm-3 (LOC models). Such models are relatively successful in reproducing the observed strength of most optical emission lines, including the [OIII] lines. However, LOC models have not considered calculating the density sensitive [ArIV] l4711/l4740 doublet ratio. The reason is that the [ArIV] lines are very weak. In a recent paper on this subject, I could rely on only 7 high excitation Seyfert 2’s observed by Koski in 1978! However, I recently had the surprise of discovering that the SDSS S7 Survey could measure [ArIV] in about 45 nuclei! Of the 22 high excitation Seyfert 2’s of the S7 sample, the [ArIV] ratios in 18 objects favour plasma densities that do not extend beyond ~50,000 cm-3. Assuming such densities, we evaluate that the temperature of the NLR must lie around 13,500K, which is inferred from the [OIII] l4363/l5007 ratio that standard photoionization models have difficulty in reproducing. We are currently exploring different solutions to the temperature problem. |
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