Temperature Discrepancy with Photoionization Models of the Narrow-Line Region

Using published work on the narrow-line region of Active Galactic Nuclei, a comparison is carried out among the [O_III] λ4363Å/5007Å ratios (R_OIII) observed in quasars, Seyfert 2's and the spatially resolved ENLR plasma. Using the weak [Ar_IV] λ4711Å/λ4740Å doublet observed by Koski (1978) among Seyfert 2's, we find evidence of a Narrow-Line Region (NLR) populated by low density emission clouds (≲ 10⁴ cm^-3). After considering calculations of the [Ar_IV] and [O_III] ratios that assume a powerlaw distribution of plasma densities, no evidence of collisional deexcitation is found. The plasma temperature that is inferred is 13 500 °K, which is problematic to reproduce with standard photoionization calculations. The simplest interpretation for the near coincidence of the R_OIII ratios among the ENLR and Seyfert 2 measurements (R_OIII ≃ 0.017) is that the low density regime applies to both plasmas.