Constraints on the Densities and Temperature of the Seyfert 2 Narrow Line Region

Different studies have reported the so-called temperature problem of the narrow line region (NLR) of active galactic nuclei (AGNs). Its origin is still an open issue. To properly address its cause, a trustworthy temperature indicator is required. We propose that the weak [ArIV] 4711,40A doublet is the appropriate tool for evaluating the density of the high excitation plasma. We subsequently made use of the recent S7 survey sample to extract reliable measurements of the weak [ArIV] doublet in 16 high excitation Seyfert 2s. As a result we could derive the plasma density of the NLR of our Seyfert 2 sample and compare the temperature inferred from the observed [OIII] (4363A/5007A) ratios. It was found that 13 Seyfert 2s cluster near similar values as the [OIII] (4363A/5007A) ratio, at a mean value of 0.0146+-0.0020. Three objects labeled outliers stand out at markedly higher [OIII] values (> 0.03). If for each object one assumes a single density, the values inferred from the [ArIV] doublet for the 13 clustering objects all lie below 60,000 cm-3, indicating that the [OIII] (4363A/5007A) ratios in these objects is a valid tracer of plasma temperature. Even when assuming a continuous power-law distribution of the density, the inferred cut-off density required to reproduce the observed [ArIV] doublet is in all cases < 1E5.1 cm-3. The average NLR temperature inferred for the 13 Seyfert 2s is 13,000+-703 K, which photoionization models have difficulty reproducing. Subsequently we considered different mechanisms to account for the observed [OIII] ratios. For the three outliers, a double-bump density distribution is likely required, with the densest component having a density > 1E6 cm-3.