TEMPERATURE DISCREPANCY with PHOTOIONIZATION MODELS of the NARROW-LINE REGION

Abstract

Using published work on the narrow-line region of active galactic nuclei, a comparison is carried out among the [O iii] λ4363Å/λ5007Å (ROIII) ratio observed in quasars, Seyfert 2’s and the spatially resolved ENLR plasma. Using the weak [Ar iv] λ4711Å/λ4740Å doublet ratio 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 power law distribution of plasma densities, no evidence of collisional deexcitation is found. The plasma temperature inferred is 13 500 °K, which is problematic to reproduce with standard photoionization calculations. The simplest interpretation for the near coincidence of the ROIII ratios among the ENLR and Seyfert 2 measurements (ROIII ' 0.017) is that the low density regime applies to both plasmas.